## Motivation and Context
The MCP specification requires that when a server receives a request containing
a session ID that is no longer valid, it MUST respond with HTTP 404 Not Found.
`handle_delete` was unconditionally calling `cleanup_session` and returning 200
even for nonexistent session IDs.
Ref: https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management
> The server MAY terminate the session at any time, after which it MUST respond
> to requests containing that session ID with HTTP 404 Not Found.
## How Has This Been Tested?
Added tests for DELETE with invalid session ID, and lifecycle tests for
delete-then-POST and delete-then-DELETE scenarios.
## Breaking Change
DELETE requests with an invalid session ID now return HTTP 404 "Session not found"
instead of HTTP 200.
However, this change is considered a bug fix because it brings the behavior into
compliance with the MCP specification.
abc7d6c7ea