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.

- Fix textareaRef any type in dialog-provider.tsx
- Extract PROVIDER_DISPLAY_NAMES to shared display-names.ts module
- Extract createGoogleVertexLoader helper to reduce code duplication
- Fix test to clear all Google env vars that could affect provider loading

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

# Conflicts:
#	packages/opencode/src/cli/cmd/auth.ts
#	packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx
#	packages/opencode/test/provider/provider.test.ts

# Conflicts:
#	packages/opencode/src/cli/cmd/auth.ts
#	packages/opencode/src/cli/cmd/tui/component/dialog-model.tsx
#	packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx

# Conflicts:
#	packages/opencode/src/cli/cmd/tui/component/dialog-provider.tsx

# Conflicts:
#	packages/opencode/src/provider/provider.ts
#	packages/opencode/test/preload.ts