feat(openai): add API Route compatibility helper

[DennyHo0917]

Summary

Adds API Route support to the existing OpenAI-compatible LLM integration via openai.LLM.with_api_route().

API Route exposes an OpenAI-compatible endpoint, so this follows the same pattern already used by helpers such as with_nebius(), with_openrouter(), with_cerebras(), and others.

Disclosure: I am affiliated with API Route. This PR adds my own service as a supported OpenAI-compatible provider.

Changes

• add openai.LLM.with_api_route()
  • defaults to https://global.api-route.com/v1
  • reads API_ROUTE_API_KEY
  • allows overriding model, api_key, base_url, and the standard OpenAI-compatible LLM options
  • keeps the model type as str so the integration does not depend on a fast-changing static model catalog
• add hermetic unit coverage for:
  • default endpoint/model and environment-variable API key
  • explicit model/base URL/API key overrides
  • missing API key validation

Usage

from livekit.plugins import openai

llm = openai.LLM.with_api_route(
    model="claude-sonnet-4-6",
)

with:

export API_ROUTE_API_KEY="..."

No new dependency or standalone plugin is required because API Route implements the OpenAI-compatible chat completions API.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.