fix(driver-galaxy): resolve Medium code-review finding (Driver.Galaxy-009)

Fix two resource-management bugs in StartDeployWatcher / BuildDefaultHierarchySource:
(a) Replace the discarded `_ = StartAsync(...)` with an explicit task variable that
    surfaces any synchronous InvalidOperationException (called-twice guard) rather than
    silently swallowing it.
(b) Change both StartDeployWatcher and BuildDefaultHierarchySource to use ??= on
    _ownedRepositoryClient so the first client created (by whichever path runs first)
    is reused by the second path, preventing a second GalaxyRepositoryClient from being
    created and the first from leaking past the driver's lifetime.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.Galaxy/findings.md

@@ -153,13 +153,13 @@
 | Severity | Medium |
 | Category | Error handling & resilience |
 | Location | `GalaxyDriver.cs:354-371` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `StartDeployWatcher` launches the watch loop with `_ = _deployWatcher.StartAsync(CancellationToken.None)` — a fire-and-forget with a discarded `Task`. `StartAsync` can throw synchronously (`InvalidOperationException` if already started); the discard masks that programming error. Separately, `StartDeployWatcher` builds an `_ownedRepositoryClient` purely for the watcher when discovery has not run yet — if `DiscoverAsync` later runs, `BuildDefaultHierarchySource` overwrites `_ownedRepositoryClient` with a second client, leaking the first (only the latest reference is disposed in `Dispose`).
 
 **Recommendation:** Await `StartAsync` (it completes synchronously after scheduling) or at least observe its result. Reuse a single `GalaxyRepositoryClient` across the deploy watcher and the hierarchy source instead of letting `BuildDefaultHierarchySource` clobber the field — guard the assignment or build the client once in `InitializeAsync`.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — (a) replaced `_ = _deployWatcher.StartAsync(...)` discard with an explicit variable + `IsFaulted` check so any synchronous throw from `StartAsync` (e.g. called-twice `InvalidOperationException`) propagates rather than being silently swallowed; (b) changed both `StartDeployWatcher` and `BuildDefaultHierarchySource` to use `_ownedRepositoryClient ??=` so a client built by the watcher is reused by discovery instead of being overwritten and leaked — only one `GalaxyRepositoryClient` instance is now created and disposed.
 
 ### Driver.Galaxy-010
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -445,17 +445,21 @@ public sealed class GalaxyDriver
 
         // Reuse the lazily-built repository client (DiscoverAsync constructs it on demand).
         // If discovery hasn't run yet, build the client here so the watcher has a target.
-        if (_ownedRepositoryClient is null)
-        {
-            _ownedRepositoryClient = MxGateway.Client.GalaxyRepositoryClient.Create(
-                BuildClientOptions(_options.Gateway));
-        }
+        // Driver.Galaxy-009 fix: guard with ??= so if BuildDefaultHierarchySource later runs
+        // it reuses this client rather than overwriting the field and leaking the first instance.
+        _ownedRepositoryClient ??= MxGateway.Client.GalaxyRepositoryClient.Create(
+            BuildClientOptions(_options.Gateway));
 
         var source = new GatewayGalaxyDeployWatchSource(_ownedRepositoryClient);
         _deployWatcher = new DeployWatcher(source, _logger);
         _deployWatcher.OnRediscoveryNeeded += (_, args) => OnRediscoveryNeeded?.Invoke(this, args);
 
-        _ = _deployWatcher.StartAsync(CancellationToken.None);
+        // StartAsync schedules the background loop and returns Task.CompletedTask immediately.
+        // It throws InvalidOperationException synchronously if called twice (programming error).
+        // Driver.Galaxy-009 fix: don't discard the return value — observe any synchronous throw.
+        var startTask = _deployWatcher.StartAsync(CancellationToken.None);
+        // The task is already completed (StartAsync is synchronous); surface any synchronous fault.
+        if (startTask.IsFaulted) startTask.GetAwaiter().GetResult();
     }
 
     /// <inheritdoc />
@@ -1032,20 +1036,10 @@ public sealed class GalaxyDriver
     /// </summary>
     private IGalaxyHierarchySource BuildDefaultHierarchySource()
     {
-        var gw = _options.Gateway;
-        var clientOptions = new MxGatewayClientOptions
-        {
-            Endpoint = new Uri(gw.Endpoint, UriKind.Absolute),
-            ApiKey = ResolveApiKey(gw.ApiKeySecretRef),
-            UseTls = gw.UseTls,
-            CaCertificatePath = gw.CaCertificatePath,
-            ConnectTimeout = TimeSpan.FromSeconds(gw.ConnectTimeoutSeconds),
-            DefaultCallTimeout = TimeSpan.FromSeconds(gw.DefaultCallTimeoutSeconds),
-            StreamTimeout = gw.StreamTimeoutSeconds > 0
-                ? TimeSpan.FromSeconds(gw.StreamTimeoutSeconds)
-                : null,
-        };
-        _ownedRepositoryClient = GalaxyRepositoryClient.Create(clientOptions);
+        // Driver.Galaxy-009 fix: reuse a client that StartDeployWatcher may have already
+        // created (??=) rather than always overwriting the field and leaking the first
+        // instance. Both paths produce equivalent clients from the same options.
+        _ownedRepositoryClient ??= GalaxyRepositoryClient.Create(BuildClientOptions(_options.Gateway));
         return new TracedGalaxyHierarchySource(
             new GatewayGalaxyHierarchySource(_ownedRepositoryClient), _options.MxAccess.ClientName);
     }