M3 R3.2 SHIPPED: AddHistoricalValuesAsync — historical backfill writes over gRPC (live-validated)

Public HistorianClient.AddHistoricalValuesAsync(tag, values) inserts non-streamed original
(backfill) values for an existing tag over the 2023 R2 gRPC front door. The pure-managed SDK
wrote a value and read it back live (gated test AddHistoricalValuesAsync_OverGrpc_WritesAndReadsBack
PASSED against the real server).

- HistorianGrpcHistoricalWriteOrchestrator: write-enabled (0x401) session ->
  RetrievalService.GetTagInfosFromName (resolves the per-tag GUID = the tag-info TypeId, and
  registers the tag on the session) -> HistoryService.AddStreamValues("ON" buffer) per sample.
- HistorianHistoricalValue (public record: TimestampUtc, Value, OpcQuality=192).
- gRPC-only: non-RemoteGrpc transports throw ProtocolEvidenceMissingException (the 2020 WCF
  non-streamed write is architecturally blocked, D2).
- Float value encoding only (the captured type); other types rejected by the serializer.

275 unit tests pass; the new gated live write/read-back test is green against the 2023 R2 server.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

tests/AVEVA.Historian.Client.Tests/HistorianGrpcIntegrationTests.cs

@@ -173,6 +173,40 @@ public sealed class HistorianGrpcIntegrationTests
         Assert.All(result.Attempts, a => Assert.False(a.Succeeded));
     }
 
+    [Fact]
+    public async Task AddHistoricalValuesAsync_OverGrpc_WritesAndReadsBack()
+    {
+        string? host = Environment.GetEnvironmentVariable("HISTORIAN_GRPC_HOST");
+        // Gated additionally on a dedicated sandbox-tag env var so this WRITE test never runs by
+        // accident — set HISTORIAN_WRITE_SANDBOX_TAG to an existing Float tag you are happy to write
+        // backfill samples to. M3 R3.2: HistoryService.AddStreamValues ("ON" buffer).
+        string? sandboxTag = Environment.GetEnvironmentVariable("HISTORIAN_WRITE_SANDBOX_TAG");
+        if (string.IsNullOrWhiteSpace(host) || string.IsNullOrWhiteSpace(sandboxTag)
+            || string.IsNullOrEmpty(Environment.GetEnvironmentVariable("HISTORIAN_USER")))
+        {
+            return;
+        }
+
+        HistorianClient client = new(BuildOptions(host));
+
+        // A backfill sample at a fixed historical second, with a distinctive value.
+        DateTime stamp = new DateTime(DateTime.UtcNow.Year, 1, 2, 3, 4, 5, DateTimeKind.Utc);
+        const double expected = 222.5;
+        bool wrote = await client.AddHistoricalValuesAsync(
+            sandboxTag!,
+            [new HistorianHistoricalValue(stamp, expected)],
+            CancellationToken.None);
+        Assert.True(wrote);
+
+        // Read the window around the sample back and confirm it landed.
+        List<HistorianSample> samples = [];
+        await foreach (HistorianSample s in client.ReadRawAsync(sandboxTag!, stamp.AddMinutes(-1), stamp.AddMinutes(1), maxValues: 16, CancellationToken.None))
+        {
+            samples.Add(s);
+        }
+        Assert.Contains(samples, s => s.NumericValue is { } v && Math.Abs(v - expected) < 0.01);
+    }
+
     private static HistorianClientOptions BuildOptions(string host)
     {
         string? user = Environment.GetEnvironmentVariable("HISTORIAN_USER");