fix(opencode): use public Copilot host for github.com

[dajiaohuang]

Issue for this PR

Closes #45302

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Older or enterprise OAuth records can contain enterpriseUrl set to github.com. The Copilot URL helper treated every non-empty enterprise URL as a GHE host and produced the invalid copilot-api.github.com endpoint.

This keeps github.com on the public api.githubcopilot.com host while preserving copilot-api. for real GHE deployments. Supersedes the closed #33662.

How did you verify your code works?

• Added a regression test that failed against dev with the wrong models URL, then passed with the fix.
• bun test test/plugin/github-copilot-models.test.ts (8 passed)
• bun typecheck from packages/opencode
• Repository pre-push typecheck (30/30 tasks)
• Prettier check and git diff --check

Screenshots / recordings

Not applicable; this is an OAuth endpoint fix.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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