feat: Add n1n.ai as a model provider

[roomote]

This PR attempts to address Issue #8657 by adding n1n.ai as a new model provider in Roo Code.

What Changed

• Added n1n.ai as an OpenAI-compatible provider with base URL https://n1n.ai/v1/
• Implemented dynamic model fetching to support 400+ available LLMs
• Added type definitions, API handler, and model fetcher for n1n provider
• Updated provider settings schema and UI components to include n1n
• Added n1n to router models configuration for dynamic provider support

Technical Details

n1n.ai provides an OpenAI-compatible API that gives access to 400+ large language models through a single API key configuration. The implementation:

• Extends the existing OpenAiHandler for API compatibility
• Uses dynamic model fetching via the models endpoint
• Follows established patterns used by other dynamic providers (OpenRouter, DeepInfra)
• Includes proper error handling and graceful fallback

Testing

• All existing tests pass
• Updated test expectations to include n1n provider in router models

Related Issue

Closes #8657

Feedback and guidance are welcome!

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Important

Adds n1n.ai as a new model provider with dynamic model fetching and integrates it into the existing system.

• Behavior:
  • Adds n1n.ai as a new model provider with base URL https://n1n.ai/v1.
  • Implements dynamic model fetching for 400+ models via getN1nModels() in n1n.ts.
  • Extends OpenAiHandler in N1nHandler to support n1n.ai API.
• Configuration:
  • Updates provider-settings.ts to include n1n in dynamicProviders and providerNames.
  • Adds n1nSchema to providerSettingsSchemaDiscriminated and providerSettingsSchema.
  • Updates modelIdKeys and modelIdKeysByProvider to include n1nModelId.
• Integration:
  • Adds N1nHandler to buildApiHandler() in index.ts.
  • Updates webviewMessageHandler.ts to handle n1n model fetching.
  • Includes n1n in MODELS_BY_PROVIDER in provider-settings.ts.

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[ellipsis-dev]

Consider logging additional details from the Zod safeParse result when parsing fails (e.g. logging parsed.error) to aid in debugging schema mismatches.

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[roomote]

The getModel() method always returns n1nDefaultModelInfo instead of using dynamically fetched models from the cache. This means the 400+ models from n1n.ai won't have accurate metadata (context windows, pricing, vision support, etc.).

The fetcher in src/api/providers/fetchers/n1n.ts is implemented and integrated, but N1nHandler never uses it. Compare with DeepInfraHandler.getModel() which properly uses cached models:

const info = this.models[id] ?? deepInfraDefaultModelInfo

N1nHandler should follow the same pattern - store fetched models and use them in getModel().