Fix visualizer server startup failures

[Aming9303]

What changed

• retry server startup only for EADDRINUSE, with a 100-port limit
• propagate other WebSocket and HTTP startup exceptions to setup()
• replace blocking lock polling with timeout-aware awaitable futures
• stop the WebSocket server even before a browser connects and clean up test servers reliably
• support port 0 so tests can request OS-assigned ports
• remove the no-longer-needed pytest-timeout dependency

Tests

• python -m pytest pylabrobot/visualizer/visualizer_tests.py -q (26 passed)
• python -m ruff check pylabrobot/visualizer/visualizer.py pylabrobot/visualizer/visualizer_tests.py
• python -m ruff format --check pylabrobot/visualizer/visualizer.py pylabrobot/visualizer/visualizer_tests.py
• python -m mypy pylabrobot/visualizer/visualizer.py

Resolves #1224

[rickwierenga]

thanks for the pr!

please green the ci

[Aming9303]

Fixed the Python 3.9 mypy failure by treating server.sockets as an iterable rather than indexing it. The focused type check now passes, along with all 26 visualizer tests, Ruff, and format checks.

[rickwierenga]

thanks!