Create Pipeline

Create and execute a texture generation pipeline.

Provide at least one of these three input modes:

1. Existing session: Provide session_id (from /upload-usd).
2. S3 reference: Provide s3_uri, downloads from S3 server-side.
3. File upload: Provide usd_file, creates new session.

When multiple sources are supplied, selection follows the legacy precedence session_id > s3_uri > usd_file and lower-priority fields are ignored. A selected S3 source is admitted only when its exact bucket name is listed in TA_S3_ALLOWED_BUCKETS; an empty allowlist rejects every client S3 input before session-manager or S3 I/O.

Optionally provide material_textures_json to specify per-material texture prompts, blend opacity, and per-prim overrides.