chore: update xgopilot model to claude-4.6-opus

[xgopilot]

Requested by @luoliwoshang

This PR updates the xgopilot configuration to use the claude-4.6-opus model.

Summary

• Updated .github/xgopilot.yml to use claude-4.6-opus instead of claude-4.5-opus

Closes #1632

[luoliwoshang]

The Token price is same with opus-4.5 , LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (9c8b6b3) to head (d14faa5).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1633   +/-   ##
=======================================
  Coverage   91.13%   91.13%           
=======================================
  Files          45       45           
  Lines       12117    12117           
=======================================
  Hits        11043    11043           
  Misses        897      897           
  Partials      177      177           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.