chore(comm): delete dead IntegrationCallRequest routing (Gitea #32)

'Pattern 4: Integration Routing' — RouteIntegrationCallAsync →
SiteEnvelope(IntegrationCallRequest) → SiteCommunicationActor integration
handler — was plumbed end to end but connected at neither end: no
producer (zero callers) and no handler (AkkaHostedService never
registered LocalHandlerType.Integration). It was an early scaffold the
architecture routed around — the brokered External→Central→Site→Central
round-trip is served by the Inbound API's routed-site-script path (the
RouteTo* verbs, driven from CommunicationServiceInstanceRouter), which is
live, tested, and shares IntegrationTimeout.

Decision (#32): delete. Removed the IntegrationCall{Request,Response}
messages, RouteIntegrationCallAsync, the SiteCommunicationActor receive
block + _integrationHandler field + LocalHandlerType.Integration, and the
four tests that covered them (2 actor, 2 message-contract, 1 dispatcher-
reject, 1 mapper-reject). KEPT IntegrationTimeout — it is the live
timeout for the RouteTo* verbs. Updated the exclusion-narrative comments
(proto/mapper/dispatcher), design §4, the components doc timeout table,
and marked the known-issue RESOLVED.

Full solution build clean (0/0); Communication 634 + Host 421 green.
Net -172/+20 across 14 files. Not in the gRPC proto (was deliberately
excluded there), so no wire-format change.

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteCommandDtoMapper.cs

@@ -34,13 +34,6 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 /// server (T1B.2) and the central transport (T1B.3) can share one implementation
 /// without a project-reference cycle.
 /// </para>
-/// <para>
-/// <b>Excluded by design:</b> <c>IntegrationCallRequest</c>. It is the 29th
-/// command on <c>SiteCommunicationActor</c>'s receive table but is dead at both
-/// ends — no production code registers the integration handler, so the site
-/// always answers "Integration handler not available". See
-/// <c>docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md</c>.
-/// </para>
 /// <para><b>Null conventions.</b> proto3 has no presence for scalars, so:</para>
 /// <list type="bullet">
 ///   <item>