fix(historian-gateway): map alarm severity numerically; dispose provisioner's gRPC channel

Driver.Historian.Gateway-001 (High): AlarmEventMapper wrote the alarm's
Severity wire property as the .NET enum name (e.g. "High"), but
EventMapper's read side int.TryParse()s that same property and silently
falls back to the minimum severity on any unparseable value - so every
alarm event's severity round-tripped through this module's own mappers
collapsed to 1 (Low), contradicting AlarmHistorianEvent.Severity's own
documented contract ("mapped to Historian's numeric priority"). Now
mapped through a numeric priority mirroring the existing
NativeAlarmProjector.MapSeverity convention (200/500/700/900).

Driver.Historian.Gateway-002 (Low): GatewayTagProvisioner never disposed
its IHistorianGatewayClient, unlike its GatewayHistorianDataSource /
GatewayAlarmHistorianWriter siblings. Now implements IAsyncDisposable so
the DI-owned singleton's gRPC channel closes gracefully at shutdown.

Both TDD: new/corrected tests confirmed RED against the pre-fix code.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayTagProvisioner.cs

@@ -17,7 +17,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway;
 ///     as <see cref="HistorianProvisionResult.Failed"/> and returns. The method never throws and never
 ///     logs tag values, hostnames, or credentials.
 /// </remarks>
-public sealed class GatewayTagProvisioner : IHistorianProvisioning
+public sealed class GatewayTagProvisioner : IHistorianProvisioning, IAsyncDisposable
 {
     /// <summary>
     ///     Storage rate (ms) stamped on every provisioned analog tag. The gateway's <c>EnsureTags</c> path
@@ -104,4 +104,13 @@ public sealed class GatewayTagProvisioner : IHistorianProvisioning
             return new HistorianProvisionResult(requests.Count, Ensured: 0, Skipped: skipped, Failed: definitions.Count);
         }
     }
+
+    /// <summary>
+    ///     Disposes the underlying gateway client (and its gRPC channel). The DI container owns this
+    ///     provisioner as a singleton (<c>AddHistorianProvisioning</c>), so this fires once at host
+    ///     shutdown — closing the channel gracefully, matching the read (<see cref="GatewayHistorianDataSource"/>)
+    ///     and alarm-write (<see cref="GatewayAlarmHistorianWriter"/>) paths' dispose ownership.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public ValueTask DisposeAsync() => _client.DisposeAsync();
 }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/Mapping/AlarmEventMapper.cs

@@ -1,5 +1,6 @@
 using Google.Protobuf.WellKnownTypes;
 using ZB.MOM.WW.HistorianGateway.Contracts.Grpc;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Mapping;
@@ -10,6 +11,25 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Mapping;
 /// </summary>
 internal static class AlarmEventMapper
 {
+    /// <summary>
+    /// Maps <see cref="AlarmSeverity"/> to the numeric OPC UA Part 9 priority (1-1000) carried in the
+    /// wire event's <c>Severity</c> property, per the <see cref="AlarmHistorianEvent.Severity"/>
+    /// contract ("Mapped to Historian's numeric priority on the sink side"). The read side
+    /// (<see cref="EventMapper.ToHistoricalEvent"/>) <c>int.TryParse</c>s this same property, so it
+    /// must stay a numeric string — the enum's <c>.ToString()</c> name (e.g. "High") is NOT
+    /// parseable and used to silently collapse every read-back severity to the minimum (1). The
+    /// values mirror the existing native-alarm convention
+    /// (<c>ZB.MOM.WW.OtOpcUa.Runtime.Drivers.NativeAlarmProjector.MapSeverity</c>).
+    /// </summary>
+    private static string ToNumericSeverity(AlarmSeverity severity) => severity switch
+    {
+        AlarmSeverity.Low => "200",
+        AlarmSeverity.Medium => "500",
+        AlarmSeverity.High => "700",
+        AlarmSeverity.Critical => "900",
+        _ => "500",
+    };
+
     /// <summary>Maps an alarm historian event to a gateway event.</summary>
     /// <param name="alarm">The driver-agnostic alarm event.</param>
     /// <returns>The gateway wire event ready for <c>SendEvent</c>.</returns>
@@ -36,7 +56,7 @@ internal static class AlarmEventMapper
             historianEvent.Properties["AlarmId"] = alarm.AlarmId;
         historianEvent.Properties["AlarmName"] = alarm.AlarmName;
         historianEvent.Properties["EventKind"] = alarm.EventKind;
-        historianEvent.Properties["Severity"] = alarm.Severity.ToString();
+        historianEvent.Properties["Severity"] = ToNumericSeverity(alarm.Severity);
         historianEvent.Properties["User"] = alarm.User;
         historianEvent.Properties["Message"] = alarm.Message;
         if (alarm.Comment is not null)

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/GatewayTagProvisionerTests.cs

@@ -149,6 +149,27 @@ public sealed class GatewayTagProvisionerTests
         Assert.Equal(1, result.Failed); // non-blocking: no throw
     }
 
+    /// <summary>
+    /// Driver.Historian.Gateway-002: unlike its siblings (<c>GatewayHistorianDataSource</c>,
+    /// <c>GatewayAlarmHistorianWriter</c>), <see cref="GatewayTagProvisioner"/> had no dispose path at
+    /// all — the gRPC channel <c>GatewayHistorian.CreateProvisioner</c> opens was never closed. The
+    /// provisioner is registered as a process-lifetime DI singleton
+    /// (<c>AddHistorianProvisioning</c>), and the built-in container disposes any singleton instance
+    /// it creates that implements <see cref="IAsyncDisposable"/>/<see cref="IDisposable"/> — regardless
+    /// of the registered service-interface type — so implementing it here is sufficient for a clean
+    /// shutdown with no other wiring change.
+    /// </summary>
+    [Fact]
+    public async Task DisposeAsync_disposes_underlying_client()
+    {
+        var fake = new FakeHistorianGatewayClient();
+        var p = Provisioner(fake);
+
+        await ((IAsyncDisposable)p).DisposeAsync();
+
+        Assert.Equal(1, fake.DisposeCallCount);
+    }
+
     [Fact]
     public async Task Ensured_count_reflects_successful_results()
     {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Tests/Mapping/AlarmEventMapperTests.cs

@@ -23,12 +23,41 @@ public sealed class AlarmEventMapperTests
         Assert.Equal("A1", e.Properties["AlarmId"]);
         Assert.Equal("HiHi", e.Properties["AlarmName"]);
         Assert.Equal("Activated", e.Properties["EventKind"]);
-        Assert.Equal("High", e.Properties["Severity"]);
+        // Severity is the AlarmHistorianEvent.Severity XML doc contract: "Mapped to Historian's
+        // numeric priority on the sink side" — NOT the .NET enum name. EventMapper.ParseSeverity on
+        // the read side int.TryParse()s this same property, so a non-numeric string (the old bug:
+        // alarm.Severity.ToString() == "High") silently always read back as the minimum severity (1).
+        Assert.Equal("700", e.Properties["Severity"]);
         Assert.Equal("operator1", e.Properties["User"]);
         Assert.Equal("ack note", e.Properties["Comment"]);
         Assert.Equal("Temp high", e.Properties["Message"]);
     }
 
+    /// <summary>
+    /// Driver.Historian.Gateway-001 regression: the write side (<see cref="AlarmEventMapper"/>) and
+    /// the read side (<see cref="EventMapper"/>) must agree on the wire representation of severity —
+    /// a numeric OPC UA Part 9 priority string, matching the convention already used elsewhere in the
+    /// codebase for the same enum (<c>NativeAlarmProjector.MapSeverity</c>: Low=200/Medium=500/
+    /// High=700/Critical=900). Round-tripping through both mappers must reproduce the numeric severity,
+    /// not collapse to the minimum every time.
+    /// </summary>
+    [Theory]
+    [InlineData(AlarmSeverity.Low, (ushort)200)]
+    [InlineData(AlarmSeverity.Medium, (ushort)500)]
+    [InlineData(AlarmSeverity.High, (ushort)700)]
+    [InlineData(AlarmSeverity.Critical, (ushort)900)]
+    public void Severity_round_trips_through_EventMapper_as_numeric_priority(AlarmSeverity severity, ushort expected)
+    {
+        var a = new AlarmHistorianEvent("A1", "Area/Line/Pump1", "HiHi", "LimitAlarm",
+            severity, "Activated", "m", "operator1", null,
+            new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc));
+
+        var wire = AlarmEventMapper.ToHistorianEvent(a);
+        var readBack = EventMapper.ToHistoricalEvent(wire);
+
+        Assert.Equal(expected, readBack.Severity);
+    }
+
     [Fact]
     public void Null_comment_is_omitted_not_null()
     {