Add MiniMax provider support

[octo-patch]

What this PR does

Adds MiniMax to the provider registry and LLM factory with support for MiniMax-M3 and MiniMax-M2.7. MINIMAX_BASE_URL selects the global or China endpoint and routes requests through the OpenAI-compatible or Anthropic-compatible client.

The provider guide documents model context, modalities, thinking behavior, pricing tiers, and all four endpoint combinations. Unit tests capture real client requests and verify the resulting request paths for each endpoint.

Supersedes #13 with a clean commit history.

Type

• Bug fix
• New architecture
• Enhancement to existing architecture
• Tests
• Documentation
• CI / tooling
• Other

Verification

• pytest -q (289 passed, 37 skipped)
• ruff check src/ tests/
• ruff format --check src/ tests/
• python -m build
• mkdocs build

Checklist

• Tests pass locally (pytest -q)
• ruff check src/ tests/ is clean
• Docs updated for the user-facing configuration
• Commit message follows the Conventional Commits format