fix(socket): handle missing user env - #2927

Summary

• initialize websocket user env parsing with an empty dict when the client sends no userEnv payload
• keep required user env validation on the intended ConnectionRefusedError path
• update socket tests that previously pinned the NameError/UnboundLocalError behavior

Validation

• uv run --no-sync ruff check chainlit/socket.py tests/test_socket.py
• uv run --no-sync ruff format --check chainlit/socket.py tests/test_socket.py
• uv run --no-sync pytest tests/test_socket.py

Note: local pytest required temporary empty chainlit/frontend/dist and chainlit/copilot/dist directories because importing chainlit.server expects built UI directories.

Summary by cubic

Fix WebSocket user env parsing to default to an empty dict when the client sends no payload, while keeping required-key validation. This avoids NameError/UnboundLocalError and raises ConnectionRefusedError only when required vars are missing.

• Bug Fixes
  • Initialize user_env_dict = {} in chainlit.socket.load_user_env when userEnv is absent.
  • Update tests to expect {} when no keys are required and ConnectionRefusedError when required keys are missing.

^{Written for commit df30c9b. Summary will update on new commits. Review in cubic}