fix: preserve Copilot catalog variants

[he-yufeng]

Summary

• keep existing catalog variants when refreshing GitHub Copilot models from the /models endpoint
• still let the endpoint-provided variants override matching catalog keys
• keep message-API models on their provider-specific variant path so stale Copilot variants are not carried across

Fixes #30092.

To verify

• bun --cwd packages\opencode test test\plugin\github-copilot-models.test.ts
• bunx prettier --check packages\opencode\src\plugin\github-copilot\models.ts packages\opencode\test\plugin\github-copilot-models.test.ts
• bun --cwd packages\opencode typecheck
• bunx oxlint packages\opencode\src\plugin\github-copilot\models.ts packages\opencode\test\plugin\github-copilot-models.test.ts

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[rekram1-node]

This is expected, we trust the api here.