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/Sessions/SessionManager.cs

@@ -365,11 +365,19 @@ public sealed class SessionManager : ISessionManager
         // rather than adopting them).
         //
         // For the same reason the loop itself is NOT bound to cancellationToken: a cancelled
-        // ParallelOptions token stops dispatching the remaining sessions entirely, whereas the
-        // sequential drain this replaced let every remaining session fail its graceful close fast
-        // and still kill its worker. The token is passed to the graceful close instead, which
-        // preserves that behavior — a host stop deadline turns the drain into a kill sweep rather
-        // than into a leak.
+        // ParallelOptions token stops dispatching the remaining sessions entirely, so a stop
+        // deadline would leave the untried tail neither closed nor killed. Note this FIXES a leak
+        // the sequential drain also had rather than restoring its behavior: there the kill fallback
+        // ran on the caller's cancelled token, and KillWorkerAsync's entry
+        // ThrowIfCancellationRequested threw out of the loop on the first session — zero kills, not
+        // "fail fast and still kill". The token is passed to the graceful close only, and the kill
+        // runs on CancellationToken.None, so a host stop deadline turns the drain into a kill sweep
+        // rather than into a leak.
+        //
+        // The asymmetry with CloseExpiredLeasesAsync (whose ParallelOptions IS token-bound) is
+        // intentional: that sweep is periodic maintenance whose missed sessions are picked up by
+        // the next pass and, ultimately, by this drain. This drain is terminal — nothing runs after
+        // it — so it must not be abandoned partway.
         await Parallel.ForEachAsync(
             _registry.Snapshot(),
             new ParallelOptions { MaxDegreeOfParallelism = MaxParallelSessionCloses },