Add Seedance 2.5 desktop wire contracts - #1889

Summary

• Add Seedance 2.5 Preview, Full, and Ultra at the desktop Tauri command boundary.
• Carry the variants through router and generation-event mappings, persisted task metadata, completion events, and TypeScript mirrors.
• Keep the canonical wire identifiers consistent: seedance_2p5_preview, seedance_2p5, and seedance_2p5_u.
• Reject unsupported alternate spellings.

Why

The router supported the models, but the desktop request/event/task contracts did not represent all three variants end to end. That left deserialization and task/event metadata boundaries incomplete.

Validation

• Focused desktop Rust tests: 6/6 passed.
• Task-model enum tests: 8/8 passed.
• Frontend enum/event tests: 2/2 passed.
• git diff --check and contribution-scope scans pass.

Origin and scope

This change was isolated from a broader downstream integration and reduced to a generic, self-contained upstream contribution. Private deployment/release-specific material and artifacts are intentionally excluded.