fix(cli): align auth plugin selection

[iam-brain]

Summary

• align CLI auth plugin selection with ProviderAuth last-wins behavior
• cover selection logic with focused unit tests
• guard non-string provider values to mirror prior CLI behavior

Test Plan

• bun test --cwd packages/opencode test/cli/auth-plugin-selection.test.ts
• bun test --cwd packages/opencode
• bun run --cwd packages/opencode script/build.ts
• manual: CLI auth login now correctly selects the custom plugin for openai installed from https://github.com/iam-brain/opencode, following the same logic that the TUI does.

Fixes #10898

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

Based on my search, I found one potentially related PR:

PR #11058: fix: allow user plugins to override built-in auth plugins
#11058

This PR appears related because it also deals with auth plugin behavior and user plugins overriding built-in plugins, which aligns with the "last-wins behavior" mentioned in PR #11076.

All other results are the current PR (#11076) itself or unrelated auth/plugin features from earlier dates.

[iam-brain]

Didn't realize #11058 was PR'ed for the same issue. Their implementation is much simpler, whereas this implementation looks to recreate the logic used in the TUI fromEntries() approach.

[iam-brain]

#11058 has been merged, woohoo! Closed as this is no longer necessary.