fix: inherit provider loaders for provider aliases

[raf1hh]

Issue for this PR

Closes #18486

Type of change

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

What does this PR do?

Fix custom providers configured with auth_provider so they inherit the referenced provider's SDK options and model loader behavior.

Before this change, built-in auth/plugin loaders were only applied to the source provider id and there is no way to reuse built-in sdk auth/model-loading behavior.

This change applies plugin auth loaders to aliased providers and reuses model/vars loaders from the referenced provider, so config like this now works:

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "my-github-copilot": {
      "name": "My GitHub Copilot",
      "auth_provider": "github-copilot",
      "npm": "@ai-sdk/github-copilot"
    }
  }
}

With this change, my-github-copilot behaves like a GitHub Copilot-backed provider for auth/model loading, while still keeping its own provider id and config entry instead of overriding github-copilot directly.

Without this fix, using a custom id here is limiting because the aliased provider does not inherit the built-in loader behavior.

How did you verify your code works?

I cherry-picked this onto current upstream dev, verified the PR diff is limited to this fix, and ran bun typecheck from packages/opencode.

Screenshots / recordings

Not a UI change.

Checklist

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