#11464: feat(models): add forward-compat fallback for gemini-3-pro-image

Added a forward-compat fallback for the google-antigravity/gemini-3-pro-image model using gemini-3-pro-high as a template. This ensures the model is available even if it's missing from the underlying SDK catalog. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a forward-compat model resolver in `src/agents/pi-embedded-runner/model.ts` to synthesize `gemini-3-pro-image` for the `google-antigravity` / `google-gemini-cli` providers by cloning the first available template model (`gemini-3-pro-high`, then `gemini-3-pro-preview`, then `gemini-3-pro`) from the local `ModelRegistry`. The resolver is invoked in `resolveModel()` after existing Codex and Anthropic forward-compat checks, and before the generic provider-config fallback, so missing catalog entries can still be selected when explicitly requested by ID. <h3>Confidence Score: 5/5</h3> - This PR looks safe to merge with minimal risk. - Change is small and self-contained: it only adds a narrowly-scoped forward-compat resolver for a single model ID and integrates it into the existing resolveModel fallback chain. No behavioral changes occur for other providers/models unless the exact missing ID is requested. - No files require special attention <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->