feat: add google_cloud provider (Vertex AI) for Gemini models

[cloetzi]

Adds a google_cloud provider for Gemini models on Google Cloud Vertex AI (now called Gemini Enterprise Agent Platform).

Why not the official gem

google-cloud-ai_platform-v1 is the only official Ruby gem, and neither of its transports works well under fibers:

• gRPC (its default): blocks the Async reactor. 5 concurrent calls took 5.04s instead of ~1s.
• REST: streaming starts one thread per stream. That thread leaks, still holding its socket, when the caller stops early with break or Async::Stop.

This provider instead uses its own Net::HTTP client, like Gemini::Client, with googleauth for tokens. Vertex's Gemini request/response JSON matches the Gemini Developer API, so GoogleCloud subclasses Gemini and only changes the transport and URL paths.

Fiber-safe auth

On Cloud Run, GKE and GCE, googleauth gets tokens through google-cloud-env, which spots re-entry by comparing Thread.current. When several fibers refresh a token at once, it raises a false ThreadError: deadlock. The client puts every googleauth call behind a process-wide Mutex, which works per fiber, and shares one set of Application Default Credentials (ADC) across clients.

Checked against a fake metadata server, 5 concurrent fibers:

• With the lock: 0 errors, and the run took about as long as one call.
• Without the lock: 8 errors.

A unit test fails if the lock is removed.

Regions

Location is set on the client. The default is "global", and the docs say plainly that it has no data-residency guarantee. They show how to bind a location with a config.google_cloud.client Proc or with a provider subclass per location.

Commits

1. Extract Riffer::Wire::SSE, a shared SSE decoder that Gemini now uses.
2. GoogleCloud::Client with fiber-safe token refresh.
3. google_cloud provider for Gemini models, with VCR cassettes recorded against a real project.
4. Docs.

Follow-ups

• Claude models on Vertex, stacked on this PR in feat/google-cloud-claude. It's waiting on Model Garden access to record cassettes.
• Mapping tags to Vertex labels.
• Sending gs:// files by URL.

🤖 Generated with Claude Code

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

• Autopilot · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

_{Comment @coderabbitai help to get the list of available commands.}