Remove WebSocket transport from code-mode hosts (#40692)

## What changed

- Accept only `http://` and `https://` gRPC endpoints for app server
  `--code-mode-host` connections.
- Limit the standalone code-mode host to stdio and gRPC listeners.
- Remove the WebSocket session provider, dual-WebSocket negotiation, transport
  lanes, and related dependencies and tests.
- Keep coverage for shared and prewarmed gRPC hosts and reject WebSocket host
  URLs in CLI parsing tests.

GitOrigin-RevId: 8792604e79ffd54ce776074a4dc3f7d55e63752a