[breaking] Google deprecates the Gemini 3 image-generation previews

gemini-3.1-flash-image-preview and gemini-3-pro-image-preview were announced May 28, 2026 for shutdown June 25, 2026. Replacements per the deprecations table: gemini-3.1-flash-image and gemini-3-pro-image respectively. Effective: 2026-06-25. Affected: google/gemini-3.1-flash-image-preview, google/gemini-3-pro-image-preview. Severity: breaking. Verification: verified.