feat: add xAI (Grok) as AI provider (#1209)

# Add xAI (Grok) Provider Support

## Overview
This PR adds support for xAI's Grok models as an AI provider, focusing
on coding-optimized models.

## Changes Made

### Provider Configuration (`language_model_helpers.ts`)
- Added xAI to `MODEL_OPTIONS` with 3 coding-focused models:
  - `grok-code-fast-1`: Fast, economical coding model (256k context)
  - `grok-4`: Most capable flagship model (256k context)
  - `grok-3`: Powerful coding model (131k context)

<img width="805" height="592" alt="image"
src="https://github.com/user-attachments/assets/a99b9495-e90e-40f3-a772-be9807b24501"
/>


<img width="805" height="653" alt="image"
src="https://github.com/user-attachments/assets/aad7b333-ee74-457a-b5b7-5d20bd54d7e0"
/>

## Dependencies
- Requires `@ai-sdk/xai` package (already imported)
- Uses existing provider pattern and infrastructure


## Why xAI for Coding?
xAI's Grok models have shown impressive results in coding benchmarks:
- Trained on high-quality programming datasets reflecting real-world
tasks
- Excels at agentic coding workflows with fast reasoning capabilities
- Strong performance across multiple programming languages (TypeScript,
Python, Java, Rust, C++, Go)
- Achieved 70.8% on SWE-Bench-Verified using internal evaluation
- Optimized for rapid iteration in development environments
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Adds xAI (Grok) as a provider so users can pick Grok coding models in
the app. Integrates provider config, client wiring, and schema updates.

- **New Features**
- Added xAI provider with env var mapping (XAI_API_KEY) and provider
metadata.
- Exposed models: grok-code-fast-1 (256k), grok-4 (256k), grok-3 (131k).
  - Hooked up get_model_client to use @ai-sdk/xai (createXai).
  - Included "xai" in validation schemas and model options.

- **Migration**
  - Set XAI_API_KEY to enable xAI.

<!-- End of auto-generated description by cubic. -->

---------

Co-authored-by: Will Chen <willchen90@gmail.com>

src/ipc/utils/get_model_client.ts

@@ -1,6 +1,7 @@
 import { createOpenAI } from "@ai-sdk/openai";
 import { createGoogleGenerativeAI as createGoogle } from "@ai-sdk/google";
 import { createAnthropic } from "@ai-sdk/anthropic";
+import { createXai } from "@ai-sdk/xai";
 import { createVertex as createGoogleVertex } from "@ai-sdk/google-vertex";
 import { azure } from "@ai-sdk/azure";
 import { createOpenRouter } from "@openrouter/ai-sdk-provider";
@@ -212,6 +213,16 @@ function getRegularModelClient(
         backupModelClients: [],
       };
     }
+    case "xai": {
+      const provider = createXai({ apiKey });
+      return {
+        modelClient: {
+          model: provider(model.name),
+          builtinProviderId: providerId,
+        },
+        backupModelClients: [],
+      };
+    }
     case "google": {
       const provider = createGoogle({ apiKey });
       return {