Add Olmo3 tool parser

[anthonyhchan]

Summary

• Adds a tool-call parser for Olmo 3 models (e.g.
  mlx-community/Olmo-3-7B-Instruct-4bit), which emit calls as
  <function_calls>name(arg=value, ...)</function_calls> with newline-separated
  multiple calls and JSON literals (null / true / false) for argument values.
• Wires auto-detection: _infer_tool_parser selects "olmo3" when the chat
  template contains <function_calls>

Implementation notes

• mlx_lm/tool_parsers/olmo3.py mirrors pythonic.py. Differences:
  • No surrounding [ ] brackets on each call.
  • Per-line matching (re.MULTILINE, no DOTALL) so multiple calls on
    separate lines are parsed independently and returned as a list.
  • _coerce translates null / true / false before falling back to
    ast.literal_eval, since Olmo 3 emits JSON literals rather than Python ones.

Test plan

• python -m pytest tests/test_tool_parsing.py -v — all 6 tests / 24 subtests pass.
• load(Path(".../Olmo-3-7B-Instruct-4bit")) returns a wrapper with
  tool_call_start == "<function_calls>", tool_call_end == "</function_calls>",
  and a non-None tool_parser