Changes

PR #2092 by Resistor52: feat(openai): add GrokAI search grounding via xAI Responses API

• Added support for GrokAI search grounding via xAI's Responses API, fixing HTTP 422 errors caused by a hardcoded OpenAI web_search_preview tool name in buildResponseParams.
• Introduced two new fields to openai_compatible.ProviderConfig: WebSearchToolName (to override the default web search tool name) and EnableXSearch (to append xAI's x_search tool when search is enabled).
• Both new fields default to empty/false, preserving full backwards compatibility for all existing providers.
• GrokAI is pre-configured with WebSearchToolName set to "web_search" and EnableXSearch set to true, enabling grounded search results with real source URLs via fabric -V GrokAI --search.
• Added tests in openai_test.go covering the new override paths and confirming default provider behavior remains unchanged.