FromHandlerType routing: create listeners for separated handler chains, not just the saga (#3041)

[jeremydmiller]

Closes #3041.

Reproduction

From the reporter's repo: RabbitMQ + UseConventionalRouting(NamingSource.FromHandlerType) + DisableConventionalLocalRouting() + MultipleHandlerBehavior.Separated, with a message handled by both a saga (a Start method) and a regular handler. Only the saga ran; the regular handler never received the message.

I reproduced it broker-free by inspecting the SagaChain for the shared message type:

chain type      : SagaChain
chain.Handlers  : [ReproSaga.Start]          // the saga only
chain.ByEndpoint: [ReproRegularHandler]      // the regular handler is a SEPARATED chain

(The reporter's own diagnostic — runtime.RoutingFor(messageType).Routes — points at the sender/exchange side; the actual missing piece is on the listener side, which the endpoint/queue diagnostics surface: only the saga's queue gets created.)

Root cause

Under MultipleHandlerBehavior.Separated, when a message has multiple handlers and one is a saga, only the saga stays in chain.Handlers — the sibling handlers are moved to separated chains in chain.ByEndpoint. MessageRoutingConvention.DiscoverListeners' FromHandlerType branch iterated only chain.Handlers, so it created a handler-named listener queue for the saga but none for the separated regular handler. With conventional local routing disabled, that handler had no queue/listener and never received the message. (The Separated branch handles ByEndpoint, but FromHandlerType takes the earlier if and never reaches it.)

Fix

The FromHandlerType branch now also creates a handler-named listener for each chain.ByEndpoint chain — registering the endpoint on that chain and marking it sticky, mirroring what the Separated branch already does — and is entered when there are only ByEndpoint chains too. One small change in MessageRoutingConvention.

Tests

New RabbitMQ regression Bug_3041_saga_and_handler_with_handler_type_naming (mirrors the repro): asserts both the saga's and the regular handler's handler-named queues exist and have active listeners. Verified it fails (regular-handler queue missing) without the fix.

• RabbitMQ conventional-routing + separated suites: 34.
• MessageRoutingTests: 25. CoreTests saga/separated: 66.

No version bump — main is unpublished 6.4.4 (latest tag V6.4.1); rides the pending release.

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