fix(utils): report free port search exhaustion

[RerankerGuo]

PR type

• Bug Fix
• New Feature
• Document Updates
• More Models or Datasets Support

PR information

find_free_port() returned the last candidate even when every bind attempt
failed. Deploy and app argument initialization could therefore accept an
occupied port and only fail later when starting the server.

This change:

• returns immediately after a successful bind, preserving the existing path;
• raises a clear OSError after exhausting the candidate range;
• prevents retries near port 65535 from scanning invalid port numbers;
• validates invalid starting ports and retry counts.

The change is limited to port selection. It does not reserve the returned port
or alter the existing bind interface.

Experiment results

Before:

find_free_port(30000, retry=3) -> 30002

when all three mocked bind attempts raised OSError.

After:

OSError: No free port found in range [30000, 30003)

Verification:

.venv/bin/python tests/run.py --test_dir tests/utils --pattern test_port_utils.py
SUCCESS (Runs=3, success=3)

.venv/bin/pre-commit run --files swift/utils/utils.py tests/utils/test_port_utils.py
All hooks passed