fix(provider): ignore GITHUB_TOKEN when activating github-copilot

[aniruddhaadak80]

Issue for this PR

Closes #44113

Type of change

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

What does this PR do?

The models.dev catalog lists GITHUB_TOKEN as an env var for github-copilot, so the generic env activation loop treats any GITHUB_TOKEN (often set for GitHub tooling) as a Copilot credential and loads the provider with source env. That bypasses the Copilot OAuth plugin, model discovery, and endpoint routing, which sends models like gpt-5.6-sol to /chat/completions where they fail.

This skips github-copilot in the env activation loop so the provider only activates through its OAuth plugin flow. Generic env activation for all other providers is unchanged.

How did you verify your code works?

• Added a regression test: with GITHUB_TOKEN set, github-copilot no longer appears in loaded providers (bun test test/provider/provider.test.ts -t GITHUB_TOKEN passes; it fails without the fix).
• Ran bun run typecheck and the full test/provider/provider.test.ts suite locally.

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potentially related PR:

Related PR:

• fix(opencode): require OAuth for GitHub Copilot #44114 - fix(opencode): require OAuth for GitHub Copilot

This PR is related because it's addressing the same issue (#44113) from a different angle. PR #44114 appears to focus on enforcing OAuth for GitHub Copilot, while PR #44231 (the current PR) specifically fixes the problem by skipping GITHUB_TOKEN env activation for the github-copilot provider. These are both working to prevent the provider from being incorrectly activated via environment variables when GITHUB_TOKEN is set.

[aniruddhaadak80]

The unit test and typecheck workflows are in action_required state - could a maintainer approve the workflow runs for this first-time contribution? Local verification: the regression test passes (and fails without the fix), full provider suite and typecheck are clean.