fix(fal): harden Hunyuan3D contracts

[JustinPerea]

Summary

• re-verify the Hunyuan3D V3 text-to-3D and image-to-3D nodes against the current official FAL schemas and pricing pages
• reject text prompts over FAL's 1,024-character limit before queue submission
• route both fixed endpoints through isolated graph-node copies so internal endpoint_id state is never persisted
• add one gold two-node contract exemplar, two request fixtures, fixture-backed parity tests, and synchronized FAL skill/provider references
• correct stale guidance: seed is response metadata on these endpoint slugs, not an accepted request input

Why

Phase 2 provider auditing still had both Hunyuan3D nodes as explicit contract gaps. The runtime mapping was otherwise current, but prompt length failed only at the provider boundary, fixed wrappers mutated graph params, and the repository documentation incorrectly described seed as an available provider input.

Impact

This closes the structural and freshness audit for both Hunyuan3D V3 nodes without changing their public node definitions. The provider inventory moves from 23 to 25 exemplar-backed nodes and from 48 to 46 remaining gaps.

Validation

• backend: 1,168 tests passed with two existing AsyncMock warnings
• frontend: clean npm ci; 394 tests, lint, and production build passed
• focused Hunyuan/fixture suite: 16 tests passed
• node contracts: 142 definitions passed
• contract inventory: 71 nodes, 25 exemplar-backed, 46 gaps
• generated model reference and git diff --check: passed
• mirrored Hunyuan FAL skills: byte-identical
• staged sensitive-filename and credential-prefix scans: passed

No paid FAL generation was run. Existing build warnings remain for the large bundle chunk and lottie-web direct eval.