Post apidatasets versions preprocess

POST /datasets/.../preprocess — open-shape preprocessing parameters.

The schema of parameters is owned by each preprocessing module (MR 3+). The runner in MR 2 will validate per-module.

Two layered size caps:

• PARAMETERS_MAX_BYTES (from VIZMR2 review): caps the encoded JSON of parameters as a whole — a guard against a runaway payload that bypasses per-format validation and bloats PreprocessingJob.parameters once the runner activates.
• MAX_RAW_TEXT_BYTES (this MR — VIZMR3): caps the Phase-1 raw_text carrier specifically. The tabular module reads file contents from parameters['raw_text'] as a stop-gap until the S3-streaming swap, so this cap matters at MUCH larger sizes than the general parameters cap. Tunable via the DATAERAI_MAX_RAW_TEXT_BYTES env var so dev fixtures can override.