Fix MiniMax M2 nullable tool arguments

[Mr-Neutr0n]

Summary

• preserve non-null MiniMax M2 tool arguments when their schema also allows null
• continue mapping explicit null, none, and nil values to Python None
• add regression coverage for nullable string and integer arguments

Why

The converter checked whether null appeared anywhere in a parameter's allowed types and returned None before inspecting the actual value. As a result, values such as "weather" and "10" were discarded for nullable parameters.

Tests

• uv run python -m unittest tests.test_tool_parsing (6 passed)
• uvx pre-commit run --files mlx_lm/tool_parsers/minimax_m2.py tests/test_tool_parsing.py (Black and isort passed)