feat: add Novita OpenAI-compatible provider

[Alex-wuhu]

Summary

• add novita as a first-class provider type in providers package
• introduce NovitaProvider that reuses OpenAI provider behavior with default endpoint https://api.novita.ai/openai
• wire Novita into provider exports and provider manager switch
• update OpenAI provider internals to honor runtime provider label (openai or novita)
• add unit test for Novita endpoint routing and response provider label

Verification

• attempted: npm test -- src/tests/novita-provider.test.ts
• result: failed with vitest not found because test dependency is not installed in this workspace

[Alex-wuhu]

CI still failing after 3 fix attempts. Leaving for manual review.