[breaking] Google deprecates the Imagen 4 GA models in favor of Gemini image generation

All three imagen-4.0 GA models were announced June 15, 2026 for shutdown August 17, 2026. The deprecations table lists gemini-3.1-flash-image as the replacement for all three — consolidating image generation under Gemini-native models. Effective: 2026-08-17. Migration target: google/gemini-3.1-flash-image. Affected: google/imagen-4.0-generate-001, google/imagen-4.0-ultra-generate-001, google/imagen-4.0-fast-generate-001. Severity: breaking. Verification: verified.