## 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a session ID that is no longer valid, it MUST respond with HTTP 404 Not Found.
The `handle_get` path already returned 404 correctly, but `handle_regular_request`
(the POST path) was returning 400 "Invalid session ID" instead.
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 a test for POST requests with an invalid session ID, verifying that the
response is 404 with "Session not found" error message. All existing tests pass.
## Breaking Change
POST requests with an invalid session ID now return HTTP 404 "Session not found"
instead of HTTP 400 "Invalid session ID". Clients that match on the 400 status
code or the old error message will need to be updated.
However, this change is considered a bug fix because it brings the behavior into compliance with
the MCP specification.
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