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

Cargo.lock

@@ -1318,6 +1318,7 @@ dependencies = [
  "dirs 7.0.0",
  "futures-util",
  "reqwest",
+ "serde_json",
  "tokio",
  "tokio-tungstenite 0.21.0",
  "tokio-util",