feat: add xAI web search provider

[mcostasilva]

Summary

• Add native xAI/Grok web search through the Responses API.
• Support API keys and SuperGrok OAuth with token refresh.
• Add citation handling, provider tests, and documentation updates.

Why pass a custom fetch function?

OpenCode’s SuperGrok OAuth provider exposes a dummy API key because authentication happens inside a custom fetch wrapper. Since the xAI adapter creates its own Responses API client, the OAuth resolver passes that wrapper through the shared client configuration. This allows token refresh and bearer-token injection without changing API-key authentication.

Why extract credential attachment helpers?

Adding xAI OAuth handling to loadResolutions would exceed the 20-statement limit. The existing ChatGPT and Copilot credential logic was moved unchanged into focused helpers; provider selection behavior remains the same.

Validation

• bun run check
• bun run build
• Live-tested with Grok 4.6

[emilsvennesson]

Thanks for the PR!