Fix conventional routing endpoint discovery in FindResources

[jeremydmiller]

Summary

• Re-enables endpoint discovery from routing conventions during FindResources() (originally fixed in Routing conventions need to apply to the stateful resource model operations #513, then reverted due to regression)
• Fixes the regression by clearing the route cache after discovery so routes with null Sender created during WithinDescription mode are not reused for actual message sending
• Adds Azure Service Bus emulator testing scaffolding (Testcontainers)

Background

WolverineSystemPart.FindResources() needs to discover all transport endpoints (exchanges, queues) so they can be provisioned/torn down as stateful resources. The original fix (GH-513) called RoutingFor() for each message type during FindResources(), which triggered MessageRoutingConvention.DiscoverSenders() and created the endpoints as a side effect.

However, RoutingFor() caches routes in _messageTypeRouting. Routes created during FindResources() have WithinDescription=true, which allows null Sender on MessageRoute. When those cached routes were later reused for actual message sending, Envelope(message, Sender) threw NullReferenceException because Sender was null.

The fix calls ClearRoutingFor() for each discovered message type after the routing loop, preserving the endpoint discovery side effect while ensuring routes get properly rebuilt with active sending agents when the host starts.

Test plan

• find_endpoints_through_conventions_as_part_of_find_resources passes
• All 6 StatefulResourceSmokeTests pass
• All 23 ConventionalRouting tests pass
• All 9 RabbitMqTransportTests pass
• end_to_end tests: 23/25 pass (2 pre-existing flaky failures from stale broker state)

Closes GH-513

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