Use correct API response format (cloudaicompanionProject) and proper
onboarding flow with tierId and metadata. Add retry logic for project
provisioning which may take time.
- Add OAuth handler with PKCE flow and local callback server
- Automatic project discovery via loadCodeAssist/onboardUser endpoints
- Store credentials with projectId for API calls
- Encode token+projectId as JSON for provider to decode
- Register as 'google-cloud-code-assist' OAuth provider