feat(minimax): add MiniMax provider support (closes #2260)

[ennajari]

Summary

• Add from_minimax() factory and from_provider("minimax/...") support targeting MiniMax's OpenAI-compatible chat completions API (https://api.minimax.chat/v1), with MINIMAX_API_KEY environment-variable fallback.
• Introduce Mode.MINIMAX_TOOLS (default) and Mode.MINIMAX_JSON, with matching response/reask handlers wired into the central dispatchers (processing/response.py, processing/function_calls.py) and streaming mode sets (dsl/iterable.py, dsl/partial.py).
• Register Provider.MINIMAX, base-URL detection in get_provider, known MiniMax model names in models.py, and an auto_client handler so from_provider("minimax/MiniMax-Text-01") works end-to-end.
• Add an integration doc, a cookbook example, a CHANGELOG.md entry, and 18 unit tests that mock the OpenAI client so no live API calls are made.

Closes #2260.

Test plan

• ruff check instructor tests/llm/test_minimax.py examples/minimax_example.py — all checks passed
• ruff format --check clean on new files
• pytest tests/llm/test_minimax.py -v — 18/18 passed
• pytest tests/dsl/ tests/test_xai_optional_dependency.py -k "not llm and not openai" — still green
• from_provider("minimax/MiniMax-Text-01", api_key="...") returns an Instructor with Provider.MINIMAX and Mode.MINIMAX_TOOLS
• get_provider("https://api.minimax.chat/v1") resolves to Provider.MINIMAX

[jxnl]

Thanks for the provider contribution. This overlaps with the newer MiniMax provider PR #2306, so I am marking this one as duplicate to keep review focused in a single place.