Fix health server initialization with None port in ModelRunner

[Copilot]

Why

Tests pass health_check_port=None to disable the health server, but ModelRunner.__init__() unconditionally calls start_health_server_thread() with the port value, causing TypeError: 'NoneType' object cannot be interpreted as an integer during socket binding.

How

Added conditional check before starting health server thread:

# Only start health server if a port is specified
if health_check_port is not None:
    start_health_server_thread(port=health_check_port, address='')

Tests

Fixes existing tests in test_runners.py and test_runners_proto.py that explicitly pass health_check_port=None.

Notes

The parameter type hint Union[int, None] already indicated this should be optional, but the implementation didn't respect None values.

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