fix(error-handling): close Theme 4 — 18 cancellation / fire-and-forget findings

Async cancellation hygiene, fire-and-forget observability, retry/shutdown
semantics, and audit-row coverage across 9 modules. Highlights:

Cancellation & lifecycle:
- AuditLog-006: SqliteAuditWriter.Dispose hops to thread pool, escaping the
  captured SyncContext that risked sync-over-async deadlock.
- AuditLog-010: SiteAuditTelemetryActor owns a private lifecycle CTS,
  threaded through drain paths instead of CancellationToken.None.
- Comm-019: CentralCommunicationActor adds lifecycle CTS for repo calls.
- Host-019: Migration StartupRetry forwards ApplicationStopping so SIGTERM
  during the bounded-retry window aborts cleanly.

Cursor / retry / counter correctness:
- AuditLog-004: SiteAuditReconciliationActor's cursor now holds at `since`
  when any row's idempotent insert is still being retried (per-EventId
  retry counter, MaxPermanentInsertAttempts=5 escape valve with LogCritical
  abandon). No more silent abandonment of permanently-failing rows.
- ConfigDB-019: Dropped the catch-and-continue on EnsureLookaheadAsync's
  SPLIT loop — by class-doc construction the catch could only mask real
  failures and let the next iteration create permanent partition holes.
- HM-017/018: HealthReportSender + CentralHealthReportLoop snapshot
  per-interval counters before sending, restore via new
  ISiteHealthCollector.AddIntervalCounters on transport failure so counts
  aren't silently lost.

Fire-and-forget / shutdown waits:
- InboundAPI-018: AuditWriteMiddleware observes faulted audit-write tasks
  via OnlyOnFaulted continuation (Warning log; response unchanged).
- SnF-024: StoreAndForwardService.StopAsync awaits in-flight retry sweep
  with a bounded SweepShutdownWaitTimeout (10s).

Leak / refactor:
- Comm-021: SiteStreamGrpcServer.SubscribeInstance wraps Subscribe in its
  own try/catch so a throw doesn't leak the relay actor or _activeStreams
  entry.
- Comm-022: VERIFIED already-closed by Comm-016's dead-code purge.
- CLI-017: BundleCommands' three subcommands delegate to ExecuteCommandAsync
  (auth-failure exit-code contract unified).

Defensive / validation:
- CLI-021: CliConfig.Load wraps file-read/JSON parse so malformed config
  prints a warning and returns defaults instead of crashing the CLI.
- Host-022: ParseLevel emits stderr one-shot warning for unrecognised
  MinimumLevel instead of silently coercing to Information.
- ESG-019: ExternalSystemClient sets HttpClient.Timeout=Infinite so the
  per-call CTS is the sole timeout source (was clipped to 100s by .NET).
- Security-020: New SecurityOptionsValidator (IValidateOptions) rejects
  empty LdapServer/LdapSearchBase with ValidateOnStart.
- DM-019: Lifecycle command timeouts now emit DisableTimedOut/EnableTimedOut/
  DeleteTimedOut audit entries (mirrors DeployFailed pattern).

Plus reconciled stale per-module Open-findings counters that had drifted
from prior sessions.

20+ new regression tests across 11 test projects; build clean; affected
suites all green. README regenerated: 75 open (was 93).

code-reviews/ExternalSystemGateway/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 5 |
+| Open findings | 4 |
 
 ## Summary
 
@@ -1065,9 +1065,11 @@ message parks) and that no exception escapes the handler.
 |--|--|
 | Severity | Medium |
 | Category | Design-document adherence |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.ExternalSystemGateway/ExternalSystemClient.cs:226,257-264`, `src/ScadaLink.ExternalSystemGateway/ServiceCollectionExtensions.cs:90-102` |
 
+**Resolution (2026-05-28):** Set `client.Timeout = Timeout.InfiniteTimeSpan` immediately after `_httpClientFactory.CreateClient($"ExternalSystem_{system.Name}")` in `ExternalSystemClient.InvokeHttpAsync`, disabling the framework's 100 s default so the per-call `CancellationTokenSource(_options.DefaultHttpTimeout)` linked CTS already built below is the sole timeout source. An operator-configured `DefaultHttpTimeout` greater than 100 s is now honoured verbatim instead of being silently clipped and misclassified as a transient "connection error". Kept the fix local to the allowed file (`ExternalSystemClient.cs`) rather than touching `ServiceCollectionExtensions.cs`/`GatewayHttpClientConfigurator`. Regression test `Call_DisablesHttpClientFrameworkTimeoutSoLongTimeoutsArentClipped` asserts the rented client starts with the framework's 100 s default and is set to `Timeout.InfiniteTimeSpan` after `InvokeHttpAsync` runs.
+
 **Description**
 
 The `-002` fix enforces the per-call timeout via a linked `CancellationTokenSource`