Close Every Code threads during bridge shutdown

[cbusillo]

Summary

• close active Every Code Discord threads during bridge shutdown instead of only closing websockets
• protect shutdown cleanup with the existing session attach lock so partially-attached sessions cannot leave orphaned threads
• add regression coverage for bound sessions, already-closed websockets, and attach-time shutdown races

Validation

• uv run python -m unittest discover -s tests -q
• uv run ruff check --diff discord_blue/doodads/every_code/bridge.py tests/test_every_code.py tests/fakes_every_code.py
• uv run ruff check discord_blue/doodads/every_code/bridge.py tests/test_every_code.py tests/fakes_every_code.py
• uv run mypy .