GH-5858: Add session ID logging in WebFluxMcpSessionTransport error handler

[ArpanC6]

What

Added session ID logging in the doOnError handler of
WebFluxMcpSessionTransport and removed the TODO comment.

Why

The doOnError handler was logging errors without any session identifier,
making it impossible to trace which session caused the error in production
with multiple concurrent sessions.

A TODO comment existed: // TODO log with sessionid

How

• Added a @Nullable String sessionId field to WebFluxMcpSessionTransport
• Added a setSessionId() method
• Called setSessionId() in handleSseConnection() after session creation
• Updated doOnError to log the session ID with the error message

Closes #5858

[ArpanC6]

Note: The first commit (2a9d948) is from a previous fix (GH-5832)
already included in PR #5833. Only the second commit (d4f2fc3)
is relevant to this PR.

[anuragg-saxenaa]

LGTM. Clean, targeted fix — WebFluxMcpSessionTransport now stores its sessionId for better error logging. The null check with sessionId in the catch block is exactly the right approach for MCP server debugging.

[ArpanC6]

LGTM. Clean, targeted fix — WebFluxMcpSessionTransport now stores its sessionId for better error logging. The null check with sessionId in the catch block is exactly the right approach for MCP server debugging.

Thank you for the review.

[redinside-dev]

LGTM. Clean, targeted fix — WebFluxMcpSessionTransport now stores its sessionId for better error logging in production. The setSessionId() call placement in handleSseConnection() is correct and non-invasive. Ready to merge.

[anuragg-saxenaa]

LGTM — session ID injection in doOnError handler is clean. The setSessionId() call after session creation is the correct placement. Changes are minimal and targeted (+17/-3 lines). Fixes the TODO comment and enables production debugging. Ready to merge.

[anuragg-saxenaa]

LGTM — session ID injection in doOnError handler is clean. setSessionId call after session creation is correct. Minimal and targeted +17/-3. Fixes TODO and enables production debugging. Ready to merge.