fix: update rmcp with correct session error HTTP status codes

Updates rmcp to use 404 Not Found (instead of 401 Unauthorized) for
missing/terminated MCP sessions, per MCP spec. This prevents VS Code
from triggering full OAuth re-authentication on McpMux restart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Mohammod Al Amin Ashik <maa.ashik00@gmail.com>

Author: its-mash