Simplify WebSocket worker connection setup

[hermabr]

Summary

• Move only authenticated WebSocket connection setup and hello/welcome negotiation into a 41-line worker_client context manager.
• Keep typed receive and result send operations as direct one-liners instead of wrapping them in a client class.
• Simplify consecutive-failure tracking and collapse duplicate completion logging in the worker loop.
• Preserve the server-pushed assignment protocol from Migrate execution coordination from HTTP to WebSockets #470.

The layer is net negative: 76 additions and 82 deletions, including 47 fewer lines in the worker loop.

Stack

• Depends on Migrate execution coordination from HTTP to WebSockets #470.

Testing

• uv run ruff check
• uv run ty check
• GITHUB_ACTIONS=true uv run pytest (613 passed, 4 skipped)