Rebased on latest main (f1ca527) after codebase changes. This is a
fresh submission after PR #22 was closed as stale.
## Why This Feature
Enables enterprise GCP deployments using existing service accounts
instead of requiring separate Gemini API keys. Many organizations
already have GCP infrastructure and prefer OAuth-based auth.
## What's New
- VertexAIDriver with full streaming support
- OAuth 2.0 token caching (50 min TTL) with auto-refresh via gcloud
- Auto-detection of project_id from service account JSON
- Security: tokens stored with Zeroizing<String>
- Provider aliases: vertex-ai, vertex, google-vertex
- Compatible with new ContentBlock::provider_metadata field
## Testing
- 6 unit tests passing
- Clippy clean (no warnings)
- End-to-end tested with real GCP service account + gemini-2.0-flash
- Both streaming and non-streaming paths verified
## Usage
export GOOGLE_APPLICATION_CREDENTIALS=/path/to/sa.json
# Set provider=vertex-ai, model=gemini-2.0-flash in config.toml
