docs: add API Route AI service configuration

[DennyHo0917]

Add an API Route configuration example to the README's existing OpenAI-compatible AI service section, following Cheaper Inference in #2755 and OrcaRouter in #2658.

I maintain API Route. This contribution was prepared with AI assistance.

The entry documents the Server URL, user-supplied key, an example model subject to account access, and the public API documentation. It reuses the existing configurable service and adds no application code or dependencies. This PR targets develop as requested in the README.

Validation on 2026-10-08:

• Authenticated /v1/models discovery returned 60 models, including the documented gpt-6.1-sol example.
• Using the Python OpenAI SDK with the same endpoint and GetDetailChangesInFile function schema from src/AI/ChatTools.cs, a sample changed-file prompt caused a tool call for example.py. After supplying a synthetic diff as the tool result, the model returned feat: update greeting message.
• git diff --check passed. The test key was not persisted or committed.

This was an API-level compatibility check, not a SourceGit desktop UI or .NET SDK run. No build was run for this seven-line documentation-only change.