feat: add MiniMax provider configuration

[octo-patch]

Reason: provider-add - configure MiniMax through the existing Claude Agent SDK extension point while preserving the current SDK/session flow.

Changes

• Add CLAUDE_PROVIDER=minimax and MINIMAX_REGION settings.
• Resolve the global https://api.minimax.io/anthropic and China https://api.minimaxi.com/anthropic endpoints automatically.
• Default MiniMax to MiniMax-M3 and allow MiniMax-M2.7 through CLAUDE_MODEL.
• Keep ANTHROPIC_BASE_URL as an explicit override and leave the SDK client/session flow unchanged.
• Document the provider, regions, endpoints, and model choices.

Verification

• pytest tests/unit/test_config.py - 31 passed.
• Focused SDK endpoint/model tests - 8 passed.
• black --check, isort --check-only, flake8, and mypy src/config/settings.py - passed.

[octo-patch]

Updated this PR with MiniMax provider selection, global and China endpoint resolution, and MiniMax-M3/MiniMax-M2.7 model configuration. I ran 31 configuration tests, 8 focused SDK tests, black, isort, flake8, and mypy.

[RichardAtCT]

Thanks for this. It is clean and well tested, but I am not merging a per-vendor block.

There are four open requests for the same thing: MiniMax here, OpenRouter in #171, DeepSeek in #208, and a generic base URL in #143. Adding one vendor at a time means a settings block per vendor forever.

I want one ANTHROPIC_BASE_URL plus model mapping that answers all four. That is the next thing I plan in this area.

If you want to pick that up instead, say so and I will point you at the shape. It also conflicts with main now, after #220 rewrote the surrounding block.