test(#3996): pin TEXT+IMAGE request construction for image-output models

Exhaustively cover wantsImageResponseModalities (added by the preceding
compatibility-guard commit): true only on supported Gemini surfaces, only
when output_capabilities.image resolves true (explicit override taking
precedence over models.dev output modalities), and only outside
title-generation/compaction utility calls. Also pin catalogue resolution,
override precedence, request construction, and the existing request-shape
diagnostics.

Author: aheritier