fix(ppt-engine): on MiniMax a deck's references to match or vary become one character reference #1014

Follow-up to #1013.

ppt_generate_image tells the deck's model that references "carries pictures to match or vary: the template's own illustration for its manner, an earlier generation for consistency, a user's photograph to restyle", up to six of them (generate_image.py#L83-L85, #L153-L160). It resolves up to six (#L371) and hands them to the host's image_generate as images (#L333).

With #1013, an image role that runs on MiniMax sends the first of those images alone, as a subject_reference of type: "character" (media_gen.py#L1000-L1004), the one kind MiniMax's image-to-image reference documents. image_generate tells its own model so on MiniMax (media_gen.py#L717-L718): its schema follows the provider the section runs on and is served per turn. The deck's does not, so on a MiniMax host the deck's model still passes the template's illustration to take its manner, or several pictures at once. The rest are dropped without a word, and the first is kept as a character, which is not what the model asked for.

A fix would:

• give ppt_generate_image a schema that follows the provider its image tool runs on, as image_generate's does (an authored to_schema, so the registry serves it per turn): on MiniMax, the description and references say one reference, kept as a character, and that a manner goes in the prompt; off MiniMax, both stay byte-identical to today's;
• decide what a second reference does on MiniMax: refused with a sentence that names the one-reference rule, or the first sent alone with the result saying the rest were not;
• carry a test in the shape of test_the_image_tool_says_what_references_do_on_the_provider_it_runs_on (tests/test_media_gen_tool.py in feat(*): run the image and video roles on MiniMax through its provider #1013) for the deck tool, served through a registry, with the section's provider switched between turns.

The host tool needs no change: only the deck engine's text and its handling of references do.