On Python 3.13 and above, `requirements.txt` installs `pypdf` instead of `PyPDF2`:

```
PyPDF2==2.12.1 ; python_version > '3.10' and python_version < '3.13'  # (Noble and below)
PyPDF==5.4.0 ; python_version >= '3.13'  # (Trixie)
```

so `odoo.tools.pdf` resolves to the `_pypdf` backend. That backend back-fills the PyPDF2 1.x camelCase API onto pypdf so callers do not have to branch on the installed library, but the alias set is incomplete in two ways:

- `cropBox` is missing entirely, while `mediaBox` is aliased.
- `mediaBox` is a getter-only property, so assigning to it raises.

## Impact

`sign/models/sign_document.py` needs all three forms, and dies on the first:

```python
box = page.cropBox if page.get('/CropBox') else page.mediaBox   # 436
...
overlay_page.mediaBox = page.mediaBox                           # 444
if page.get("/CropBox"):
    overlay_page.cropBox = page.cropBox                         # 446
```

```
File ".../sign/models/sign_document.py", line 436, in render_document_with_items
    box = page.cropBox if page.get('/CropBox') else page.mediaBox
AttributeError: 'PageObject' object has no attribute 'cropBox'
```

`render_document_with_items` is reached from `sign_request_item.sign` → `_post_fill_request_item` → `_send_completed_documents` → `_generate_completed_documents`, so **signing any document fails on Python 3.13+**.

Fixing the shim rather than the caller keeps the design the module already follows: `_pypdf2_2.py` does the same normalisation for the 2.x backend, and `sign` is written against the camelCase API on purpose.

## Why CI does not catch it

The test that covers exactly this branch, `sign/tests/test_sign_request.py::test_origin_offset_translation`, patches the backend by module path:

```python
with patch('PyPDF2._page.PageObject.add_transformation', create=True) as mock_add, \
     patch('PyPDF2._page.PageObject.cropBox', new_callable=PropertyMock, return_value=offset_box):
```

On 3.13+ `PyPDF2` is not installed at all, so the target never resolves and the branch is never exercised. A follow-up on the enterprise side should patch through the backend `odoo.tools.pdf` resolved instead of naming `PyPDF2`; that change is what would keep this from regressing.

## Verification

Reproduced and fixed on Python 3.14.4 with pypdf 5.4.0, running the exact `sign_document.py` sequence:

```
L436 read cropBox/mediaBox -> (0.0, 0.0)
L444/L446 writes -> OK
round-trip cropbox: (0.0, 0.0)
```

Before the patch, line 436 raises the `AttributeError` above.

## Forward-port

Not needed. On `master` the monkeypatch block is gone and the remaining shims carry `@deprecated("PyPDF2 1.x compatibility shims are deprecated, switch to modern API")`, so callers there are expected to move to the modern API rather than lean on these aliases. This is a 19.0-only stable-branch fix.