Fix/issue 817 idle timeout log level

[lutz-grex]

Idle keep-alive timeout is normal zombie-session cleanup, not a transport failure. This PR stops it from producing ERROR-level log lines that are indistinguishable from real transport fatals.

Motivation and Context

When LocalSessionWorker hits its configured keep-alive timeout (default 5 min), it returns WorkerQuitReason::Fatal, which the transport worker logs at tracing::error!. The SessionConfig::keep_alive docstring explicitly
frames this as a safety net for silently-dropped HTTP connections — normal lifecycle cleanup, not an error condition.

This causes every idle reap to emit:

ERROR rmcp::transport::worker: worker quit with fatal: keep alive timeout after 300000ms, when poll next session event

Operators who wire ERROR-level alerts get paged on something that is normal by design. Downstream consumers cannot filter idle reaps without also silencing real transport fatals.

Additionally, the post-exit cleanup path in close_session could fail with SessionServiceTerminated when the worker had already exited due to idle timeout, producing a second spurious error.

Closes #817

How Has This Been Tested?

• Added an integration test (test_keep_alive_timeout_does_not_emit_error_log) that spins up a Streamable HTTP server with a 200ms keep-alive, connects a client, waits for idle reap, and asserts: no ERROR-level logs are
  emitted, and a DEBUG log with IdleTimeout is present.
• Added a second integration test (test_explicit_close_on_live_session_succeeds) that verifies close_session on a still-alive worker succeeds without error.
• Existing tests pass locally (cargo test).

Breaking Changes

None. WorkerQuitReason is #[non_exhaustive], so the new IdleTimeout variant is additive. The removed LocalSessionWorkerError::KeepAliveTimeout variant was not part of the public API.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This implements Option A from #817: a dedicated WorkerQuitReason::IdleTimeout(Duration) variant that is logged at debug level alongside the other expected-exit arms (Cancelled, TransportClosed, HandlerTerminated). True
transport failures continue to flow through Fatal and keep their ERROR severity.

Changes across two commits:

1. d41d3c8 — Add WorkerQuitReason::IdleTimeout, return it from the keep-alive arm, log at debug. Remove unused KeepAliveTimeout error variant.
2. 2463369 — Swallow SessionServiceTerminated in close_session when the worker has already exited, preventing a spurious error during post-exit cleanup.