fix(document): allow render-pdf to be framed and 503 cleanly on missing PyMuPDF (#2103)

* fix(document): allow render-pdf to be framed and 503 cleanly on missing PyMuPDF

Fixes #2101.

Two related bugs in the PDF-form library preview flow:

1. SecurityHeadersMiddleware was sending X-Frame-Options: DENY and
   frame-ancestors 'none' on /api/document/{doc_id}/render-pdf, but
   static/js/documentLibrary.js embeds the response in an <iframe> for
   the library card preview. The browser blocked the load with
   ERR_BLOCKED_BY_RESPONSE, leaving the user with a blank panel.

   Extend the existing is_tool_render exemption to also cover
   /api/document/.../render-pdf. Per-document owner checks still run in
   the route handler, so the exemption is scoped the same way as the
   tool-render exemption it mirrors. /api/document/.../export-pdf is
   left untouched — it's a download (Content-Disposition: attachment),
   not an iframe embed.

2. routes/document_routes.py:render_pdf called fill_fields, which
   raises RuntimeError via _require_fitz() when the optional PyMuPDF
   dependency isn't installed. That RuntimeError bubbled out as a
   generic 500 with a cryptic 'PDF render failed' detail.

   Reuse the existing _load_pdf_viewer_fitz() helper to fail fast with
   a 503 and a user-actionable install hint (mentions
   requirements-optional.txt and AGPL-3.0), matching the convention
   used by the other PDF endpoints.

Tests cover both fixes:
- middleware headers on /api/document/.../render-pdf (iframeable, but
  X-Content-Type-Options and Referrer-Policy are still set)
- middleware headers on /api/document/.../export-pdf (must stay strict)
- middleware path matching precision (similar-but-different paths stay
  strict)
- middleware headers on /api/tools/.../render (no regression)
- middleware headers on /api/chat (no regression)
- render-pdf returns 503 with install hint when PyMuPDF is missing
- 503 is raised before any file I/O (fail-fast ordering)

* chore: address maintainer feedback on PDF previews same-origin framing and comment trimming

* chore: make render-pdf regression tests order-independent

routes/document_routes.py

@@ -1332,6 +1332,12 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
             if not pdf_path:
                 raise HTTPException(404, f"Source PDF {upload_id} not found")
 
+            # Fail fast with a clear 503 if the optional PyMuPDF dependency
+            # is missing — fill_fields/stamp_annotations will otherwise
+            # raise RuntimeError deep inside and bubble out as a 500.
+            # Mirrors the convention in _load_pdf_viewer_fitz above.
+            _load_pdf_viewer_fitz()
+
             values = parse_markdown_to_values(doc.current_content or "")
             out_path = tempfile.NamedTemporaryFile(suffix=".pdf", delete=False).name
             _to_unlink.append(out_path)