feat: add Novita AI as a model provider

[Alex-wuhu]

Summary

• Adds novitaai as a new implicit provider using Novita AI's OpenAI-compatible endpoint (https://api.novita.ai/openai)
• Auto-discovered when NOVITA_API_KEY is set in the environment
• Ships three models from Novita's catalog: moonshotai/kimi-k2.5 (default), zai-org/glm-5, minimax/minimax-m2.5
• Follows the same pattern as existing providers (moonshot, nvidia, qianfan, etc.)

Files changed

• src/agents/models-config.providers.static.ts — buildNovitaProvider() + model catalog
• src/agents/models-config.providers.ts — import, re-export, and SIMPLE_IMPLICIT_PROVIDER_LOADERS entry
• src/secrets/provider-env-vars.ts — novitaai: ["NOVITA_API_KEY"]
• src/agents/model-auth-env-vars.ts — novitaai: ["NOVITA_API_KEY"]
• src/agents/models-config.providers.novitaai.test.ts — provider tests