feat: upgrade MiniMax default model from M2.5 to M2.7

[octo-patch]

Summary

Upgrade the default MiniMax model from M2.5 to the latest M2.7 across all MiniMax-related presets.

Changes

• MiniMax CN preset (minimax-cn): Default model updated from MiniMax-M2.5 → MiniMax-M2.7
• MiniMax Global preset (minimax-global): Default model updated from MiniMax-M2.5 → MiniMax-M2.7
• Aliyun Bailian preset: Added MiniMax-M2.7 and MiniMax-M2.7-highspeed to the model list (M2.5 retained as legacy option)
• Unit tests: Added test verifying MiniMax presets default to M2.7

Why M2.7?

MiniMax-M2.7 is the latest model release with improved coding capabilities and performance. All env overrides (ANTHROPIC_MODEL, ANTHROPIC_DEFAULT_SONNET_MODEL, ANTHROPIC_DEFAULT_OPUS_MODEL, ANTHROPIC_DEFAULT_HAIKU_MODEL) are updated accordingly.

Test plan

• Unit tests pass (npm run test:unit — 56/64 pass, 8 pre-existing failures unrelated to this change)
• New test MiniMax presets default to M2.7 passes
• Manual: select MiniMax (CN) or MiniMax (Global) preset and verify default model shows M2.7

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