cli: expose harmony and gpt-oss tool parsers

[Thump604]

Supersedes #216 with the same functional diff rebased on current main.

Why this PR exists:

• I rebased the original branch locally and validated it cleanly.
• Direct maintainer push back to the original computor-org fork failed with 403 from this auth context, so the original PR could not be updated in place.

Scope:

• expose harmony and gpt-oss as valid --tool-call-parser choices in vllm-mlx serve
• factor parser construction into create_parser() so the serve CLI surface is unit-testable
• add regression coverage in tests/test_server.py

Local validation:

• python -m py_compile vllm_mlx/cli.py tests/test_server.py
• python -m black --check --fast vllm_mlx/cli.py tests/test_server.py
• pytest -q tests/test_server.py

Authorship note: this restack preserves the original implementation from #216; this PR exists only to unblock review/merge on a writable branch.