fix: handle external image relationships in MsWordDocumentBackend (#3114)

* fix: handle external image relationships in MsWordDocumentBackend

When a .docx file contains image relationships with TargetMode="External"
(common in documents saved from web browsers), accessing
`_Relationship.target_part` raises ValueError because external relationships
don't have a target part within the package.

Check `rel.is_external` before accessing `target_part`, emitting a
UserWarning with the external target URL and returning None so external
images fall through to the existing "image cannot be found" handling.

Includes test with ground truth files for a .docx with external image
references.

Fixes #3113

Signed-off-by: rongo-ms <127863751+rongo-ms@users.noreply.github.com>

* chore: upgrade dependencies in uv.lock file

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

---------

Signed-off-by: rongo-ms <127863751+rongo-ms@users.noreply.github.com>
Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>
Co-authored-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>