fix(auth): fix opencode auth provider metadata URL (#50593) - #52673

Issue for this PR

Closes #50593

Type of change

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

What does this PR do?

• Prevents bare provider identifiers (e.g. opencode auth login opencode) from being treated as enterprise well-known URLs.
• Restricts well-known enterprise auth metadata fetches to valid http:/https: URLs via isWellKnownLoginUrl. When a non-URL provider name is passed positionally, it falls through to the standard provider authentication flow rather than attempting fetch("opencode/.well-known/opencode") which throws fetch() URL is invalid.
• Validates enterprise metadata structure (auth.command and auth.env) to ensure missing configuration fails gracefully.
• Adds comprehensive regression unit tests in @opencode-ai/opencode verifying login URL validation and provider fall-through.

How did you verify your code works?

• Tested with bun test test/cli/help/help-snapshots.test.ts test/cli/providers-login-url.test.ts. All 8 tests pass cleanly.

Screenshots / recordings

N/A

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.