feat: add API Route as LLM provider

[DennyHo0917]

Summary

• add API Route as a built-in OpenAI-compatible LLM provider
• add a dedicated API Route settings tab with API key and endpoint fields
• support importing the available model list from GET /v1/models
• configure imported models for chat and tool calling through LlamaIndex OpenAILike

API Route's default endpoint is https://global.api-route.com/v1. The integration intentionally does not advertise embeddings because that capability is not part of the documented API Route interface.

Verification

• compiled the changed Python modules with python -m py_compile
• parsed both modified JSON configuration files successfully
• git diff --check passes
• full pytest suite was not run locally because the project's pytest/Poetry dependencies are not installed in this environment

Disclosure: this contribution is made for API Route, and I can help maintain the integration.