Compatibility shim. Implementation lives in llm.sdk.providers.gemini.native.

Vertex AI Imagen 3 / 4 image-generation adapter.

Endpoint:
  POST {host}/v1/projects/{project}/locations/{location}/publishers/google/models/{model}:predict
Body:
  {:instances [{:prompt "..."}]
   :parameters {:sampleCount N :aspectRatio "1:1" :seed ...}}
Response:
  {:predictions [{:bytesBase64Encoded "..." :mimeType "image/png"}]}

Auth: same GCP OAuth as vertex-gemini — token from
:request provider-options.vertex.access-token or
GOOGLE_OAUTH_ACCESS_TOKEN.

Models surfaced under :vertex-imagen include imagen-3.0-generate-002,
imagen-3.0-fast-generate-001, imagen-4.0-generate-001.

Gemini Native API transport adapter.
Handles thought signatures, streaming deltas, safety metadata.
Preserves provider-specific replay state.

Vertex AI Gemini transport adapter.

Builds on Gemini native with different auth (GCP OAuth) and endpoint
structure. Auth resolution follows the standard GCP ADC chain via
llm.sdk.gcp-auth: request opts → GOOGLE_OAUTH_ACCESS_TOKEN env →
`gcloud auth print-access-token` → GOOGLE_APPLICATION_CREDENTIALS
service-account JSON (RS256-signed JWT exchanged at
oauth2.googleapis.com/token).

Project resolution: request opts → profile quirks →
GOOGLE_CLOUD_PROJECT env → SA JSON project_id.