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.Server/Grpc/MxAccessGatewayService.cs

@@ -101,6 +101,12 @@ public sealed class MxAccessGatewayService(
         try
         {
             requestValidator.ValidateInvoke(request);
+
+            // PERF(followup): this resolve and the sessionManager.InvokeAsync below look the same
+            // session up twice (a dictionary hit each, so measured cost is negligible). Collapsing
+            // them needs a SessionManager overload taking an already-resolved GatewaySession, which
+            // would duplicate InvokeAsync's fault mapping (SessionNotFound / state checks / metrics)
+            // at a second entry point — deliberately not worth it until a profile says otherwise.
             GatewaySession session = ResolveSession(request.SessionId);
             MxCommand command = request.Command;
             BulkConstraintPlan? bulkConstraintPlan = await ApplyConstraintsAsync(
@@ -697,9 +703,13 @@ public sealed class MxAccessGatewayService(
 
             default:
                 // Only the four bulk-write kinds above reach FilterWriteBulkAsync, so this is
-                // unreachable; keep the previous behaviour (the unmodified command) rather than
-                // emitting a payload-less one if that ever stops holding.
-                return command.Clone();
+                // unreachable. It throws rather than falling back to the unmodified command,
+                // because that fallback failed OPEN: a fifth bulk-write kind added upstream
+                // without a case here would silently ship the DENIED entries to the worker while
+                // still reporting them denied to the caller. Failing loud on a kind nobody can
+                // reach today is strictly safer than a constraint bypass nobody would notice.
+                throw new UnreachableException(
+                    $"Command kind {command.Kind} reached bulk-write constraint filtering without a filter case.");
         }
 
         return filtered;