feat(providers): add AI/ML API provider preset #401

Problem

AI/ML API is OpenAI-compatible and can be configured manually in Zero, but it is not discoverable in the built-in provider catalog or setup wizard. Users must currently enter the endpoint, key environment variable, and model manually.

Requested behavior

Add a focused built-in AI/ML API provider preset using Zero's existing OpenAI-compatible path:

• catalog id / label: aimlapi / AI/ML API
• position: immediately after the recommended GitLawb OpenGateway entry
• base URL: https://api.aimlapi.com/v1
• key env: AIMLAPI_API_KEY
• default model: openai/gpt-5-chat
• bearer auth and existing OpenAI-compatible chat/tool/streaming runtime
• existing /models discovery and coding-model filtering
• runtime attribution headers scoped only to this catalog id:
  • X-AIMLAPI-Partner-ID: Gitlawb
  • X-AIMLAPI-Integration-Repo: Gitlawb/zero
  • X-AIMLAPI-Integration-Version: 1.0.0

The implementation should stay narrow: provider catalog entry, catalog/runtime regression tests, and a short README setup example. No new transport, dependency, or generic provider abstraction.

Why it belongs in Zero

Zero already supports many hosted OpenAI-compatible providers as first-class presets. A named AI/ML API entry gives users one-key access to multiple model families without requiring custom endpoint configuration, while reusing the established provider architecture.

Alternatives or workarounds considered

Users can create a custom-openai-compatible profile today, but that path is manual and does not provide branded setup visibility or stable integration attribution.

Examples, screenshots, or prior discussion

AI/ML API OpenAI SDK compatibility: https://docs.aimlapi.com/quickstart/supported-sdks
API keys: https://aimlapi.com/app/keys

I can implement and test this focused scope. @jatmn, could you confirm the scope and apply issue-approved so the PR complies with CONTRIBUTING.md?