feat: Replace python tests with native E2E test suite and fix capabilities schema

- Added native E2E test suite in stub to validate server<->stub<->nvim IPC
- Added win-nvim integration tests
- Fixed empty capabilities schema in initialize response in server and win-nvim
- Refactored server to accept dynamic MCP_PORT for test isolation
- Made server exit gracefully on port 3000 collision

stub/Cargo.toml

@@ -14,3 +14,6 @@ tracing-appender = "0.2.5"
 tracing = "0.1.44"
 tracing-subscriber = "0.3.23"
 dirs = "7.0.0"
+
+[dev-dependencies]
+serde_json = "1.0.151"