Use an async version of create_connection in flash healtcheck

[thomasjpfan]

Describe your changes

This async function used socket.create_connection which is blocking. This PR introduces an async version of socket.create_connection with ``async.create_connection` and uses that instead.

Checklists

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Compatibility checklist

Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Release checklist

If you intend for this commit to trigger a full release to PyPI, please ensure that the following steps have been taken:

• Version file (modal_version/__init__.py) has been updated with the next logical version
• Changelog has been cleaned up and given an appropriate subhead

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Note

Introduce an async port-check helper and replace the blocking socket check in Flash heartbeat with asyncio-based connectivity checks, adding targeted tests.

• Utilities (modal/_utils/async_utils.py):
  • Add async helper is_port_connection_open(host, port, timeout) using asyncio.open_connection.
• Flash (modal/experimental/flash.py):
  • Use is_port_connection_open in _FlashManager.is_port_connection_healthy instead of blocking socket.create_connection.
  • Minor adjustments: add brief sleeps while waiting, broaden exception handling to include asyncio.TimeoutError.
• Tests (test/async_utils_test.py):
  • Add tests test_is_port_connection_open and test_is_port_connection_open_failure.

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