feat: add API Route as an OpenAI-compatible provider alias

[DennyHo0917]

Summary

Add API Route as an opt-in OpenAI-compatible alias, following the Atlas Cloud (#3962) and DaoXE (#4246) patterns.

• Register api-route with https://global.api-route.com/v1 and API_ROUTE_API_KEY.
• Support automatic credential discovery/forwarding with gpt-6.1-sol as the default model.
• Reuse the existing Chat Completions transport and consecutive-system-message handling.
• Add a provider guide, navigation/table/schema entries, and a runnable code-assistant example.
• Extend existing alias, auto-selection, system-message and hermetic/live transport tests.

API Route is absent from the embedded models.dev snapshot. The existing catalog-exemption mechanism is used without editing the generated snapshot. The guide explains the missing metadata and conservative attachment handling; no pricing, context-window or multimodal claims are added.

Validation

• Windows build using the project's scripts/build.sh: passed.
• Config/provider package tests: passed with the local HTTP proxy removed for the SSRF-address test.
• Real request through TestOpenAIAliasProvider_LiveAPI/api-route: passed, streamed pong using gpt-6.1-sol.
• Authenticated GET /v1/models: 57 IDs, including the exact default ID.
• Real Chat Completions tool call: add with arguments {"a":2,"b":3}, finish_reason: tool_calls.
• git diff --check: passed. Commit carries DCO sign-off and a GitHub-verified SSH signature.

The full Windows suite and project cops are still running locally; results will be updated before this draft is marked ready. Native golangci-lint reports four findings in unchanged Windows-only tests (pkg/sandbox/options_windows_test.go, pkg/hooks/helpers_windows_test.go); a baseline comparison is in progress.

Disclosure: I am affiliated with API Route. This contribution was prepared with AI assistance and reviewed before submission.