[SECURITY BUG] mcpgateway API Endpoint Exposes Server ID Validation Gap, Echoes Prior Vulnerability

A critical security vulnerability has been identified within the mcpgateway component, where the `/servers/{id}/message` API endpoint fails to validate the provided `server_id` against the database. This flaw allows the endpoint to process requests for non-existent servers, creating a potential vector for unauthorized data manipulation or injection. The issue mirrors a class of vulnerability previously addressed in a separate endpoint, indicating a systemic pattern of validation gaps within the application's transport layer.

The bug is located in the message endpoint's handling of the path parameter. While the companion Server-Sent Events (SSE) endpoint at `/servers/{id}/sse` correctly validates the server's existence via a `server_service.get_server()` call, the message endpoint omits this crucial check. An attacker can exploit this by obtaining a valid `session_id` from a legitimate SSE connection and then directing messages to arbitrary, potentially fabricated server IDs. The presence of the required `session_id` query parameter acts as a mitigating factor, but does not fully close the security gap, as it does not bind the session to a specific, validated server instance.

This oversight reintroduces a risk profile the development team had previously worked to eliminate, as it is the same vulnerability class fixed in issue #3891 and pull request #3892 for the Streamable HTTP transport. The failure to implement consistent validation logic across related endpoints exposes the system to potential data integrity issues and could be leveraged in conjunction with other flaws. It places immediate scrutiny on the code review and testing processes for the mcpgateway API, highlighting a need for more comprehensive security audits of all data validation pathways.