chore(followups): reviewer-recommended tests, comments, and hardening from the remediation reviews

The remediation reviews approved every task but left a tail of small notes.
This lands the gateway-side half of them.

Hardening (behavior changes, all narrow):

- BuildFilteredWriteBulkCommand's unreachable default case failed OPEN: a
  fifth bulk-write kind added upstream without a filter case here would have
  shipped the DENIED entries to the worker while reporting them denied to the
  caller. It now throws UnreachableException.
- SqliteCanonicalAuditStore.ListRecentAsync no longer throws on a row it
  cannot date. The retention sweep deliberately preserves such rows (SQLite's
  datetime() yields NULL, so the DELETE never matches), which guaranteed the
  dashboard's recent-audit view would meet one eventually and lose the whole
  page to it. The row is now reported at DateTimeOffset.MinValue with every
  other column intact, behind an optional logger.
- The audit drain loop's finally now completes the channel writer alongside
  detaching the drain, so a producer that raced past the attached check takes
  the write-through branch instead of stranding its event in a buffer nobody
  reads until shutdown. TryComplete is idempotent, so StopAsync is unaffected.

Tests:

- MapCommandReply ownership (Assert.Same on the inner reply), mirroring the
  existing MapEvent ownership test.
- Redactor key-id length boundary at exactly 64 and 65 characters, pinning
  which way it fails. Nothing validates key-id length at creation, so
  docs/Diagnostics.md's "which no issued key id does" is now stated as the
  heuristic it is.
- ApiKeyFailureLimiter.Reset with a PartitionResolution whose partition was
  evicted between the Check and the Reset: inert, and clears nobody else's
  block.
- Constraint-cache concurrency stress: the cap is enforced by the inserting
  thread, so overshoot must be transient and proportional to the in-flight
  inserters, and the cache must settle at or under the cap.
- ListRecentAsync against a raw-SQL undateable row.

Comment/doc accuracy:

- EventsHubViewerRegistry.ReleaseConnection records that it relies on
  SignalR's default sequential per-connection dispatch
  (MaximumParallelInvocationsPerClient = 1).
- A PERF(followup) note on Invoke's double session resolve and why removing it
  needs a SessionManager overload.
- SessionEventDistributor: the volatile-field comment named the pump as the
  lock-free reader, but the pump's single capture point is inside _replayLock;
  the genuinely lock-free reader is SubscriberCount. OnSubscriberOverflow's
  "cannot be observed here" now excepts the DisposeAsync abandon path. The
  churn test names its ConcurrentDictionary bucket-order assumption and that a
  violation surfaces as a read timeout, not a silent pass.
- The two "restores the sequential drain's behavior" claims (SessionManager,
  docs/Sessions.md) were wrong: the sequential drain leaked too, because
  KillWorkerAsync's entry ThrowIfCancellationRequested aborted the whole loop
  on the first session for zero kills. Reworded to "fixes a leak the
  sequential drain also had", with the sweep-bound/shutdown-unbound
  ParallelOptions asymmetry explained.
- ISessionManager.ShutdownAsync's token doc: it degrades the drain to a kill
  sweep rather than cancelling it, with the bounded overrun stated.
  SessionShutdownHostedService.StopAsync records that its cancellation-logging
  branch is now unreachable.

src/ZB.MOM.WW.MxGateway.Tests/Security/Audit/ChannelAuditWriterTests.cs

@@ -219,6 +219,33 @@ public sealed class ChannelAuditWriterTests : IDisposable
             (await ListActionsAsync(factory)).OrderBy(action => action, StringComparer.Ordinal));
     }
 
+    /// <summary>
+    /// The sweep above deliberately preserves rows it cannot date, so the dashboard's recent-audit
+    /// view is guaranteed to meet one eventually. Reading must therefore be defensive: the
+    /// undateable row is reported at <see cref="DateTimeOffset.MinValue"/> with every other column
+    /// intact, rather than one bad row throwing the whole page away.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task SqliteStore_ListRecent_WithUnparseableRow_ReportsMinValueInsteadOfThrowing()
+    {
+        (SqliteCanonicalAuditStore store, AuthSqliteConnectionFactory factory) = CreateStore();
+
+        DateTimeOffset occurred = new(2026, 5, 18, 0, 0, 0, TimeSpan.Zero);
+        await store.InsertAsync(MakeEvent("dateable", occurred), CancellationToken.None);
+        await InsertRawRowAsync(factory, "undateable", "0000-not-a-timestamp");
+
+        IReadOnlyList<AuditEvent> recent = await store.ListRecentAsync(10, CancellationToken.None);
+
+        Assert.Equal(2, recent.Count);
+        Assert.Equal(occurred, recent.Single(auditEvent => auditEvent.Action == "dateable").OccurredAtUtc);
+
+        AuditEvent undateable = recent.Single(auditEvent => auditEvent.Action == "undateable");
+        Assert.Equal(DateTimeOffset.MinValue, undateable.OccurredAtUtc);
+        Assert.Equal("operator01", undateable.Actor);
+        Assert.Equal(AuditOutcome.Denied, undateable.Outcome);
+    }
+
     private static (ChannelAuditWriter Writer, AuditDrainService Drain) CreateWriterAndDrain(
         IAuditEventSink sink,
         SecurityOptions? security = null,
@@ -265,7 +292,8 @@ public sealed class ChannelAuditWriterTests : IDisposable
         await command.ExecuteNonQueryAsync(CancellationToken.None);
     }
 
-    // Reads actions straight from SQL: ListRecentAsync would throw on the unparseable timestamp.
+    // Reads actions straight from SQL so the sweep assertion depends on the DELETE alone, with no
+    // opinion from the store's read path about how an undateable row is surfaced.
     private static async Task<List<string>> ListActionsAsync(AuthSqliteConnectionFactory factory)
     {
         await using SqliteConnection connection = await factory.OpenConnectionAsync(CancellationToken.None);