Add support for authenticating to Google Vertex AI using service account
JSON credentials, with proper credential separation from regular Google
(Gemini) API keys.

Changes:
- Add service account JSON auth flow in CLI (`opencode auth login`)
- Add TUI dialog for pasting service account JSON with location input
- Update provider loaders to use stored credentials with googleAuthOptions
- Explicitly set `key: undefined` and `apiKey: null` in Vertex loaders to
  prevent credential leakage from other providers
- Rename "Vertex" to "Google Vertex AI" consistently across UI:
  - /connect menu
  - Model selection dialog
  - Status bar below text entry
- Add comprehensive tests verifying credential separation between google,
  google-vertex, and google-vertex-anthropic providers

The three providers (google, google-vertex, google-vertex-anthropic) are
now completely independent and can be configured simultaneously without
credentials leaking between them.