Add IBrokerHealthProbe + RabbitMQ implementation (CritterWatch#70)

[jeremydmiller]

Summary

Introduces an opt-in IBrokerHealthProbe contract on ITransport so monitoring layers (CritterWatch in particular) can render broker connectivity at a glance without bouncing connections. Ships the contract plus a RabbitMQ implementation; other transports follow in separate PRs.

This pairs with CritterWatch#73's bus/listener health checks for a coherent broker-monitoring story: CritterWatch drives the existing per-listener WolverineTransportHealthCheck for fine-grained endpoint health, and IBrokerHealthProbe for connection-level rollup.

What ships

• src/Wolverine/Transports/IBrokerHealthProbe.cs — IBrokerHealthProbe, BrokerHealthSnapshot record, BrokerHealthStatus enum (Unknown/Healthy/Degraded/Unhealthy). Class-level XML doc spells out the non-destructive contract: probes don't reconnect or bounce; if the connection is down they return Unhealthy rather than throw.
• src/Transports/RabbitMQ/Wolverine.RabbitMQ/Internal/RabbitMqBrokerHealthProbe.cs — partial on RabbitMqTransport so the transport itself implements IBrokerHealthProbe. Inspects IConnection.IsOpen / CloseReason, tracks reconnect counts via the existing RecoverySucceededAsync event in ConnectionMonitor, and reads TLS cert expiry from ConnectionFactory.Ssl.CertPath when configured. Initial connect is recorded as a timestamp but does not bump the reconnect counter — only genuine recoveries do. A reconnect within the last two minutes flips status from Healthy to Degraded.
• src/Testing/CoreTests/Transports/IBrokerHealthProbe_contract_tests.cs — sanity-checks the public surface (enum members, record value-equality, custom probe implementation).
• src/Transports/RabbitMQ/Wolverine.RabbitMQ.Tests/RabbitMqBrokerHealthProbe_tests.cs — boots a Wolverine host with UseRabbitMq().AutoProvision(), asserts the transport is discoverable via runtime.Options.Transports.OfType<IBrokerHealthProbe>(), probes for Healthy, then closes both connections externally and probes again for Unhealthy.
• docs/guide/messaging/transports/broker-health-probes.md — new docs page describing the contract, discovery pattern, status semantics, and per-transport support; linked from the RabbitMQ index page.

Deliberate scope cut

Azure Service Bus, Amazon SQS/SNS, Kafka, and Pulsar are not in this PR. They'll pattern-match the RabbitMQ implementation as their underlying client SDKs are surveyed; doing them all here would bloat the diff without changing the shape of the contract. The docs page lists them as Pending.

Test coverage

• Contract tests: dotnet test src/Testing/CoreTests/CoreTests.csproj --filter IBrokerHealthProbe_contract_tests — 3 tests passing on net9.0.
• RabbitMQ integration tests: dotnet test src/Transports/RabbitMQ/Wolverine.RabbitMQ.Tests --filter RabbitMqBrokerHealthProbe_tests — 3 tests passing locally on net9.0 against rabbitmq:4-management. Marked [Trait("Category", "Flaky")] to follow the project convention for tests that hit the broker.

Test plan

• dotnet build against Wolverine, Wolverine.RabbitMQ, Wolverine.RabbitMQ.Tests, CoreTests — 0 errors.
• Contract tests pass without Docker.
• Integration tests pass with Docker + RabbitMQ container running.
• CI runs the integration test on the standard RabbitMQ matrix.

🤖 Generated with Claude Code