## Summary
- Add MiniMax-M2.7 and MiniMax-M2.7-highspeed to the model selection
list (direct provider and OpenRouter)
- Set MiniMax-M2.7 as the new default model
- Retain all previous models (M2.5, M2.5-highspeed) as available
alternatives
- Update related unit tests

## Why
MiniMax-M2.7 is the latest flagship model with enhanced reasoning and
coding capabilities.

## Changes
- `src/ipc/shared/language_model_constants.ts`: Added M2.7 and
M2.7-highspeed models to the `minimax` provider section (before existing
M2.5 models) and added M2.7 to the `openrouter` section
- `src/__tests__/minimax_provider.test.ts`: Updated tests to verify all
4 models are present, M2.7 is default (first), and model ordering is
correct

## Testing
- Unit tests updated and passing
- Integration tested with MiniMax API
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Co-authored-by: PR Bot <pr-bot@minimaxi.com>