fix: close pypdfium2 page handle on error + cover classifier/OCR edge cases

Follow-up to the tiered document processor (#858), landing the round-4 review
nits the reviewer approved without:

- pypdfium2_fast: free the page handle in an outer finally so a corrupt page
  that makes get_textpage() raise can't orphan it.
- test: classify_from_text junk-text-layer path (non-zero chars, low quality,
  high ocr_frac) flags bad_text_layer -- the hot-path coverage gap.
- test: build_ocr_backend raises ValueError when the gateway M2M client_id is
  set without its token_url/secret.

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

nextcloud_mcp_server/document_processors/pypdfium2_fast.py

@@ -40,11 +40,15 @@ def _extract(content: bytes) -> tuple[str, dict[str, Any]]:
         page_texts: list[str] = []
         for i in range(len(pdf)):
             page = pdf[i]
-            textpage = page.get_textpage()
             try:
-                page_texts.append(textpage.get_text_bounded() or "")
+                textpage = page.get_textpage()
+                try:
+                    page_texts.append(textpage.get_text_bounded() or "")
+                finally:
+                    textpage.close()
             finally:
-                textpage.close()
+                # Outer finally so the page handle is freed even if
+                # get_textpage() raises on a corrupt page.
                 page.close()
         doc_meta = pdf.get_metadata_dict() or {}
     finally: