fix(raw): preserve insertbefore fragment order

[Nagi-Inaba]

Summary

• preserve the source order of multi-element XML fragments passed to raw-set --action insertbefore
• remove the reverse iteration that caused A, B, C to be written as C, B, A

Root cause

XElement.AddBeforeSelf inserts each element immediately before the same anchor while retaining previously inserted siblings. Iterating the fragment in reverse therefore persisted the reverse of the caller-supplied order.

User impact

Raw XML edits using insertbefore no longer silently reorder the supplied fragment.

Validation

Built the current source with .NET SDK 10.0.302:

dotnet build src/officecli/officecli.csproj --no-restore --nologo
Build succeeded: 0 errors

Ran the same command-level reproduction before and after the change against a disposable DOCX:

officecli create issue222.docx
officecli raw-set issue222.docx /document \
  --xpath '//w:sectPr' \
  --action insertbefore \
  --xml '<w:p><w:r><w:t>ISSUE222_A</w:t></w:r></w:p><w:p><w:r><w:t>ISSUE222_B</w:t></w:r></w:p><w:p><w:r><w:t>ISSUE222_C</w:t></w:r></w:p>'
officecli close issue222.docx
unzip -p issue222.docx word/document.xml

Expected: ISSUE222_A,ISSUE222_B,ISSUE222_C
Before:   ISSUE222_C,ISSUE222_B,ISSUE222_A
After:    ISSUE222_A,ISSUE222_B,ISSUE222_C

The build still reports one pre-existing nullable warning in ExcelHandler.SheetShift.cs:538; this change introduces no new warning.

Fixes #222