Expose Harmony/GPT-OSS tool parser in serve CLI

[krystophny]

Summary

• expose harmony and gpt-oss as valid --tool-call-parser values for vllm-mlx serve
• factor CLI parser construction so the flag surface is unit-testable
• add regression tests proving the serve CLI accepts the parser choices already registered in code

Why this is independently deployable

• CLI-only surface fix for already-registered parsers
• no batching, cache, scheduler, or API protocol change
• immediately useful for local GPT-OSS/Harmony tool-use without launcher-side hacks

Related upstream context

In upstream waybarrios/vllm-mlx, the parser building blocks already exist:

• #50 merged Harmony parser support: Add Harmony format parsers for GPT-OSS models waybarrios/vllm-mlx#50
• #53 merged GPT-OSS reasoning parsing: feat: GPT-OSS reasoning parser for channel-based token format waybarrios/vllm-mlx#53

The gap this PR fixes is narrower: those parsers were registered, but the serve CLI did not expose them consistently via --tool-call-parser.

Related ongoing work around tool-choice / tool parsing behavior upstream:

• #173 tool_choice=none: fix: honor tool_choice=none by stripping tools and suppressing parsing waybarrios/vllm-mlx#173
• #177 streaming reasoning branch tool parsing: fix: parse tool calls in streaming reasoning branch waybarrios/vllm-mlx#177
• #210 follow-up tool_choice=none: fix: respect tool_choice="none" by excluding tools from template waybarrios/vllm-mlx#210
• vllm#37433 Responses tool_choice for GPT-OSS/Harmony: [Responses API] tool_choice support (auto / required / none) for GPT-OSS vllm-project/vllm#37433

This PR does not try to solve those behavioral questions. It only exposes the parser selections cleanly.

Validation

• PYTHONPATH=/Users/ert/code/vllm-mlx /Users/ert/code/.venv/bin/python -m pytest tests/test_server.py -q
• python3 -m compileall vllm_mlx

What could still improve

• end-to-end HTTP tests covering --tool-call-parser harmony and --tool-call-parser gpt-oss
• matrix tests covering parser selection plus tool_choice=auto/none/required