feat(novita): MidJourney request shapes (mj-txt2img + 7 edit/ops)

[duanbing]

Adds 8 NovitaRequestShape variants for the Novita/PPIO /v3/async/mj-* MidJourney endpoints, consumed by RouterBase's midjourney model family.

Shapes

• MjTextToImage — maps the prompt onto text (MJ's field; MJ flags like --ar/--stylize ride inside the text). Resolves to a 4-image grid.
• MjVariation / MjUpscale / MjReroll / MjOutpaint / MjInpaint / MjRemix — operate on a prior task (task_id + image_no); they carry no top-level prompt, so build_body makes prompt optional for them. Their task_id/image_no/type/mode/scale/remix_prompt pass through the per-shape allow-list.
• MjRemoveBackground — url only.

Post-processing

• Int-coerce image_no/type/mode and float-coerce scale (RouterBase's media surface may ship them as strings; Novita's mj-* schemas are strict).
• Inpaint: wrap the mask image URL into the {url} object Novita expects (the common B/W-mask case; polygon areas deferred).

cargo check + cargo fmt clean. Single self-contained change to novita.rs.

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[duanbing]

Descoping MidJourney — closing.