Raise WebSocketDisconnect after duplicate close

[ishaanlabs-gg]

Summary

Fixes #2766.

WebSocket.send() now raises WebSocketDisconnect when the application-side websocket state is already disconnected. This matches the existing disconnect exception used elsewhere in the websocket API and leaves the receive-after-disconnect path unchanged.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

No documentation update was needed because this keeps the existing websocket exception type consistent rather than adding a new API.

Verification:

• .venv-codex/bin/python -m pytest tests/test_websockets.py::test_duplicate_close tests/test_websockets.py::test_duplicate_disconnect -q
• .venv-codex/bin/python -m pytest tests/test_websockets.py -q
• .venv-codex/bin/python -m ruff format --check --diff starlette tests
• .venv-codex/bin/python -m ruff check starlette tests
• git diff --check

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}