Fix TypeError when ModelRunner initialized with health_check_port=None

[Copilot]

Why

Tests in test_runners.py and test_runners_proto.py explicitly pass health_check_port=None to ModelRunner, but the health server thread was started unconditionally, causing TypeError: 'NoneType' object cannot be interpreted as an integer during socket binding.

How

• Added conditional check in ModelRunner.__init__ to only start health server thread when health_check_port is not None
• Preserves health probe flag initialization (is_ready, is_startup) regardless of server state, as these indicate model readiness independent of HTTP server presence

# Before: Always started, crashed with None
start_health_server_thread(port=health_check_port, address='')

# After: Conditional start
if health_check_port is not None:
    start_health_server_thread(port=health_check_port, address='')

Tests

• Validated syntax and logic correctness
• Verified existing tests that pass health_check_port=None no longer encounter TypeError
• Linting and security checks pass

Notes

• Type hint already allowed Union[int, None] - fix aligns implementation with signature
• Health probe flags remain outside conditional as they reflect model state, not server existence

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