fix: Return negotiated protocol version in initialize

The initialize method should return the protocol version requested by
the client (after validation), not the server's default protocol version.
This ensures proper protocol version negotiation per MCP spec 2025-06-18.

- Changed InitializeResult to use negotiated protocol_version
- Fixed doctest to validate correct protocol version handling
- Ensures client and server agree on the protocol version being used

Signed-off-by: Mihai Criveti <[email protected]>

Author: crivetimihai

mcpgateway/cache/session_registry.py

@@ -1251,7 +1251,7 @@ async def handle_initialize_logic(self, body: Dict[str, Any], session_id: Option
             logger.debug(f"Stored capabilities for session {session_id}: {client_capabilities}")
 
         return InitializeResult(
-            protocolVersion=settings.protocol_version,
+            protocolVersion=protocol_version,
             capabilities=ServerCapabilities(
                 prompts={"listChanged": True},
                 resources={"subscribe": True, "listChanged": True},