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/Protos/site_command.proto

@@ -17,10 +17,6 @@ import "google/protobuf/wrappers.proto";
 // place on the client and one dispatch switch on the server. A `oneof` request
 // / reply envelope preserves per-command typing inside the group.
 //
-// IntegrationCallRequest (the 29th command) is deliberately NOT here — it is
-// dead code at both ends; see
-// docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md.
-//
 // EVOLUTION RULE (same as sitestream.proto): field numbers are never reused and
 // changes are additive only. New commands take the next free oneof tag; an
 // older peer simply sees an unset oneof and answers with a clean error.