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.Commons/Messages/Integration/IntegrationCallRequest.cs

@@ -1,14 +0,0 @@
-namespace ZB.MOM.WW.ScadaBridge.Commons.Messages.Integration;
-
-/// <summary>
-/// Request routed from central to site to invoke an integration method
-/// (external system call or notification) on behalf of the central UI or API.
-/// </summary>
-public record IntegrationCallRequest(
-    string CorrelationId,
-    string SiteId,
-    string InstanceUniqueName,
-    string TargetSystemName,
-    string MethodName,
-    IReadOnlyDictionary<string, object?> Parameters,
-    DateTimeOffset Timestamp);

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Integration/IntegrationCallResponse.cs

@@ -1,12 +0,0 @@
-namespace ZB.MOM.WW.ScadaBridge.Commons.Messages.Integration;
-
-/// <summary>
-/// Response for an integration call routed through central-site communication.
-/// </summary>
-public record IntegrationCallResponse(
-    string CorrelationId,
-    string SiteId,
-    bool Success,
-    string? ResultJson,
-    string? ErrorMessage,
-    DateTimeOffset Timestamp);