Expose temperature control for Gemini image models

Gemini's image models (`gemini-2.5-flash-image`, `gemini-3-pro-image`, `gemini-3.1-flash-image`, and the Lite variant) all support a `temperature` parameter, but the app never exposed or sent it — every request used the model default. This is needed to reduce variance for multihop edits, where a chain of successive edits on the same image tends to converge to near-identical output without a way to reintroduce variation at each step.

Start by locating the image-generation request path and the handling for `gemini-2.5-flash-image`, `gemini-3-pro-image`, `gemini-3.1-flash-image`, and the Lite variant. Check how model parameters reach the request, then verify that a temperature control is exposed and sent for these models so multihop edits can reintroduce variation.

Written by the indexing model from the issue text.