fix(session): always dispose in closeGracefully

[phanikpemmaraju]

Motivation and Context

• "closeGracefully()" could skip disposing open connections when onClose threw, leading to leaked resources during server outages or abrupt disconnects.
• MCP session id is optional; when the server doesn’t return one, client flows that assumed a non-null id would throw "Session ID missing" prematurely.

How Has This Been Tested?

"Unit tests"

• closeGracefully disposes connections when onClose throws.
• Disposal exceptions are aggregated/suppressed as appropriate; error semantics preserved.

"Integration tests"

• WebFlux/SSE: server crashes mid-session → closeGracefully() best-effort local teardown still runs (existing WebFluxSseCloseGracefullyIntegrationTests).

Breaking Changes

• "None"

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

"What's changed"

• Wrap onClose.apply(sessionId) in try/finally and always dispose openConnections in the finally block.
• Add a null/absent session id guard before reconnect/close paths; treat missing id as “no remote teardown”, but still perform local cleanup.
• Improved test coverage (unit + integration).

"Why"

• Ensures robust cleanup even when the remote close fails or server is gone.
• Prevents spurious “Session ID missing” errors when servers legitimately omit a session id.

[phanikpemmaraju]

looking for reviewers to review the same.