cli: merge serve tool-call parser exposure for harmony and gpt-oss

[krystophny]

Upstream reference: waybarrios#216

Problem:
vllm-mlx serve already has parser implementations for harmony and gpt-oss, but the CLI does not expose them consistently through --tool-call-parser. That makes the surface area incomplete for local tool-calling experiments.

Path to completion:

1. Rebase the branch if needed and keep the diff limited to CLI exposure + tests.
2. Re-run tests/test_server.py.
3. Double-check that help text and parser choice validation match the real parser registry.
4. If upstream prefers a different naming or grouping for parser options, adjust the flag surface rather than defending the exact spelling.
5. Shepherd the PR as a low-risk CLI-surface fix after the correctness backlog is under control.

Acceptance criteria:

• serve --tool-call-parser exposes the already-supported parser choices consistently
• CLI regression tests cover the new values
• PR cli: expose harmony and gpt-oss tool parsers waybarrios/vllm-mlx#216 is merged or closed with a clear alternative path

[krystophny]

Fix is on fork main. Tracking upstream merge via the corresponding PR.