DoclingParseDocumentBackend.page_count() returns -1 when docling-parse fails to parse PDF page tree

### Description

`DoclingParseDocumentBackend.page_count()` can return `-1`, causing downstream failures.
This happens when `docling-parse`'s C++ backend fails to parse a PDF's page tree (QPDF exception) but `pypdfium2` succeeds.

The method logs `"Inconsistent number of pages: N!=-1"` but returns the `-1` value from `self.dp_doc.number_of_pages()` to the caller.

### Root cause

The `docling-parse` C++ layer (`pdf_decoder`) initializes `number_of_pages` to `-1`:

```cpp
// docling-parse: src/parse/pdf_decoders/document.h
pdf_decoder::pdf_decoder():
// ...
number_of_pages(-1),
// ...
```

It's only set to the real value if `qpdf_pages.getKey("/Count").getIntValue()` succeeds inside `process_document_from_file` / `process_document_from_bytesio`.
If QPDF throws, the exception is caught and logged, but `number_of_pages` remains `-1`:

```cpp
catch(const std::exception& exc)
{
LOG_S(ERROR) << "filename: " << filename
<< " can not be processed by qpdf: " << exc.what();
return false;
}
```

The Python wrapper (`docling_parse.pdf_parser.PdfDocument.number_of_pages()`) propagates this `-1` without validation. Then in `docling`:

```python
# docling/backend/docling_parse_backend.py
def page_count(self) -> int:
len_1 = len(self._pdoc) # pypdfium2 — returns correct count
len_2 = self.dp_doc.number_of_pages() # docling-parse — returns -1

if len_1 != len_2:
_log.error(f"Inconsistent number of pages: {len_1}!={len_2}")

return len_2 # returns -1 to caller
```

### Impact

In our production docling-serve deployment (15 RQ worker pods), we see ~505 of these errors in 12 hours, spread evenly across workers. The errors are document-specific (certain PDFs always trigger it) and not load-dependent.

Example log lines:
```
ERROR:docling.backend.docling_parse_backend:Inconsistent number of pages: 1!=-1
ERROR:docling.backend.docling_parse_backend:Inconsistent number of pages: 5!=-1
ERROR:docling.backend.docling_parse_backend:Inconsistent number of pages: 4!=-1
ERROR:docling.backend.docling_parse_backend:Inconsistent number of pages: 2!=-1
```

A `-1` page count causes `is_valid()` to return `False` (since `-1 > 0` is false), which likely causes the document to be rejected entirely — even though pypdfium2 can parse it fine.

### Suggested fix

In `page_count()`, handle the disagreement defensively:

```python
def page_count(self) -> int:
len_1 = len(self._pdoc)
len_2 = self.dp_doc.number_of_pages()

if len_2 < 0:
_log.warning(
f"docling-parse returned invalid page count ({len_2}) "
f"for document {self.document_hash}, "
f"falling back to pypdfium2 count ({len_1})"
)
return len_1

if len_1 != len_2:
_log.error(f"Inconsistent number of pages: {len_1}!={len_2}")

return len_2
```

Additionally, `docling-parse` should either raise on `-1` or validate `number_of_pages >= 0` after document loading (separate issue for `docling-project/docling-parse`).

### Environment

- docling-serve v1.14.0 (RQ mode, 15 GPU worker pods)
- docling 2.x (latest)
- docling-parse (latest)
- Observed on multiple distinct PDFs