Add handler.type OpenTelemetry tag for handlers and HTTP endpoints

[jeremydmiller]

Summary

Closes #2416

Tags the current System.Diagnostics.Activity with the handler type name during both message processing and HTTP request handling, providing per-handler tracing visibility in OpenTelemetry backends (Jaeger, Zipkin, Honeycomb, etc.).

• handler.type tag added to activity spans for both message handlers and Wolverine.HTTP endpoints
• message.handler tag retained on message handlers for backward compatibility (same value as handler.type)
• Tag values are memoized as string literals in Wolverine's generated code — zero runtime cost per request
• New TagHttpHandlerPolicy (IHttpPolicy) registered as a built-in policy in WolverineHttpOptions
• 3 integration tests using ActivityListener to verify tags are set correctly
• Documentation added to the "Instrumentation and Metrics" guide

Example tags

Message handler activity:

handler.type = "MyApp.Handlers.OrderPlacedHandler"
message.handler = "MyApp.Handlers.OrderPlacedHandler"

HTTP endpoint activity:

handler.type = "MyApp.Endpoints.OrderEndpoint"

Test plan

• 3 new tests in CoreTests.Runtime.handler_type_activity_tagging — all passing
• Verify HTTP endpoint tagging in Wolverine.Http.Tests

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