[M5] Add Meshy and local HTTP AI 3D provider adapters with fallback

[weiuou]

Goal

Add the first external and local-provider adapters so API-based and self-hosted image-to-3D options can be tested behind the same interface.

Scope

• Add Meshy Image-to-3D adapter using model_type=lowpoly as the default paperability-oriented mode.
• Add local_http adapter for local TripoSR/Hunyuan3D/TRELLIS-style services without coupling to a specific model repo.
• Map provider timeout, API failure, bad response, and missing model URL to provider failure metadata.
• Download or accept OBJ/GLB provider outputs and pass them into the normalization path.
• Fallback to procedural provider when external provider fails.

Acceptance Criteria

• Meshy adapter can be tested with mocked create-task, poll-task, and model-download responses.
• Local HTTP adapter can be tested with mocked model bytes.
• Provider failures do not fail the full generation task when procedural fallback succeeds.
• Task events record provider failure reason and fallback status.