feat: integrate xAI adapter

[lucianchen]

Summary

• replace the xAI stub with an OpenAI-compatible chat completions request
• add the requested xAI model list
• support dry-run short-circuiting, usage token mapping, and status/body excerpts on provider errors

Verification

• pnpm exec vitest run packages/ai/xai/src/index.test.ts
• pnpm --filter @profullstack/sh1pt-ai-xai typecheck
• git diff --check

Part of #63.