fix: Deprecated django tenant schema function replaced - #647

Replaced deprecated django tenant schema function

Correctness / robustness:
- Return the real whisper_hash in extraction_metadata (was ''), and reject
  image mode + highlight explicitly instead of silently dropping highlights.
- Store page images under {extract_dir}/{stem}/pages (per-document), not the
  per-run hash — a re-extraction now overwrites its own pages instead of
  orphaning a fresh tree in FileStorage every run.
- persist_page_images cleans up already-written pages on a mid-list failure.
- Poll loop fails closed: only explicit intermediate states continue; a
  failure/unknown/empty status raises immediately instead of hanging to the
  budget. _safe_json logs non-JSON/non-object bodies.
- verify_page_count is now live: expected count derived locally from the input
  PDF (pdfplumber); extract asserts a contiguous 1..N page set (catches
  truncation + duplicate/misnamed members) and logs skipped entries.
- Retrieve advertises accept: application/zip; submit sends Content-Type.

Layering / guard (UNS-757):
- Promote OUTPUT_MODE/image/PDF_EXTENSION/PDF_ONLY_ERROR + an is_pdf() helper
  to the shared x2text.constants (ImageOutputConstants); backend imports the
  generic surface instead of the adapter's private src.
- Move the PDF-only guard into dynamic_extractor (the single extract choke
  point, under profile_manager) so all entry points and prompt-level profile
  overrides are covered; gate on the LLMWhisperer adapter id as well as
  output_mode so other x2text adapters can't inherit the rejection.

Comments: drop ticket-id/PR provenance and the contradictory PR#536/#647
contract notes; remove the unused IMAGE_MODE_LABEL 'single source of truth'.

Tests: fix the constants-reload cross-file pollution (retry tests now pin the
real budget on helper.WhispererDefaults); add network-layer coverage
(poll success/failure/non-JSON-fast-fail/budget, mid-stream download error +
close, submit-without-hash), mid-list cleanup, image+highlight rejection, a
non-LLMWhisperer no-op, and a dynamic_extractor call-site test. sdk1: 57 pass;
backend guard: 12 pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…omments

Follow-up to the review round: remove the contradictory 'ASSUMED contract
(PR #647)' vs 'verified against PR #536' notes and the 'decision 2A' / UNS-743
provenance from the adapter helper + constants comments, keeping the factual
one-line reason (comments carry the why, not the ticket).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>