feat: add MiniMax M2.7 and M2.5-highspeed models

[octo-patch]

Summary

• Add MiniMax-M2.7 (latest flagship) and MiniMax-M2.5-highspeed (fast variant) to the built-in MiniMax provider
• List MiniMax in the supported providers section of both English and Chinese READMEs
• Add vitest testing framework with 28 unit + integration tests for provider configuration

Changes

File	Change
lib/ai/providers.ts	Add M2.7 and M2.5-highspeed models to MiniMax provider
README.md	Add MiniMax to supported providers list
README-zh.md	Add MiniMax to supported providers list (Chinese)
vitest.config.ts	New vitest configuration
package.json	Add vitest devDependency and test scripts
lib/ai/__tests__/providers.test.ts	21 unit tests for MiniMax provider config
lib/ai/__tests__/providers.integration.test.ts	7 integration tests for getModel()

Context

MiniMax recently released M2.7, their latest flagship model. This PR adds it as the top model in the existing MiniMax provider, along with the M2.5-highspeed variant for latency-sensitive workloads. The Anthropic-compatible API endpoint (api.minimaxi.com/anthropic/v1) supports all MiniMax models.

Test plan

• All 28 vitest tests pass (npx vitest run)
• Verify M2.7 model works via MiniMax Anthropic-compatible API
• Verify model appears in the provider/model selector UI

[wyuc]

Thanks for the contribution. PR #182 is adding broader MiniMax support (LLM + TTS + Image + Video) and we're using that as the main MiniMax PR. I've asked the author there to include your MiniMax-M2.7 model entry as well.

If you'd prefer, you can also open a smaller PR after #182 lands with just the model addition. Either way your work won't be lost.

A couple of notes if you do update this PR:

• The Vitest setup conflicts with test: add Vitest infrastructure with provider-config and settings-sync tests #144 which already merged. Please remove vitest.config.ts, the test files, and the vitest dependency.
• The outputWindow for M2.7 should be 131072 per MiniMax docs, not 8192.

[octo-patch]

Thanks for the heads up, @wyuc! PR #182 sounds like the better fit for broader MiniMax coverage (LLM + TTS + Image + Video). I'll close this PR. Happy to open a follow-up after #182 lands if any M2.7 model entries are still needed.