Fix idle ConnectionManager shutdown

[nekrasovp]

Summary

• cancel and await idle websocket reader tasks in the Polymarket, Kalshi, and Opinion connection managers
• let an already-started callback dispatch complete before cancellation, preserving existing delivery behavior
• make pre-start and repeated stop calls safe without consumer access to private task attributes
• add focused lifecycle regression, negative, and idempotency coverage and document the public lifecycle

Validation

• uv sync --group dev
• uv run ruff check .
• uv run mypy
• uv run pytest -q -s (58 passed)
• uv build
• uv run twine check dist/*
• uv run mkdocs build --strict
• exact commit independently verified with the same gate in a git clone --no-local checkout

Closes #9.