fix: Clean up sessions from manager when terminated via DELETE request

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Summary

This PR fixes a memory leak issue in StreamableHTTPSessionManager where sessions were not being cleaned up from the manager's internal dictionary when terminated via DELETE requests.

Problem

Previously, when clients sent DELETE requests to terminate sessions:

• The StreamableHTTPServerTransport would mark itself as terminated
• However, the StreamableHTTPSessionManager would continue to hold references to these terminated sessions in its _server_instances dictionary
• This caused memory accumulation over time in long-running servers

Solution

Added a callback mechanism to properly clean up terminated sessions:

• Added callback parameter to StreamableHTTPServerTransport constructor (on_session_terminated)
• Implemented cleanup method in StreamableHTTPSessionManager (_on_session_terminated)
• Enhanced termination flow to notify manager when sessions are terminated via DELETE
• Fixed HTTP status codes to return 404 NOT_FOUND for unknown session IDs (was 400 BAD_REQUEST)

Technical Details

1. When a DELETE request terminates a session, StreamableHTTPServerTransport._terminate_session() now calls the callback
2. The callback removes the session from StreamableHTTPSessionManager._server_instances
3. Subsequent requests with that session ID properly return 404 NOT_FOUND
4. Memory is freed for garbage collection

Test Plan

• Added comprehensive test test_session_cleanup_on_delete_request
• Verified existing session termination tests still pass
• Confirmed no regressions in broader test suite
• Validated proper HTTP status codes (404 for unknown sessions)

Backward Compatibility

✅ This change is fully backward compatible - no breaking changes to existing APIs.

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