Preserve page deletions when restoring the remaining page order

[yunfeizhu]

Fixes #21971.

After deleting trailing pages and moving the remaining pages back to their original order, PagesMapper clears the mapping even though the document is shorter. The viewer then treats the document as unchanged and downloads the original PDF, bringing the deleted pages back.

Track the original page count and only clear the mapping when both the count and order match the original document.

Add regression coverage to the existing Reorganize Pages View > Save a pdf integration suite. The tests delete one or three trailing pages through the sidebar, restore the remaining pages' order, click the main Save button, and verify the saved page selection and order. The original-download path is also captured so the regression fails an assertion instead of timing out.

Validation

• Chrome 152 and Firefox Nightly 158: all 3 save integration tests pass, including the 2 new regressions, using a TESTING=true generic build.
• With the original PagesMapper implementation, both new tests fail in both browsers because saving takes the original-download path. The existing save test still passes.
• TESTING=true npx gulp generic and npx gulp lint pass.

The full test suite has not been run locally. The regression tests follow the existing integration-test convention of checking the save payload, since actual saving is disabled in testing builds.

[Snuffleupagus]

Shouldn't this use an integration-test, similar to all the existing split/merge tests, rather than a unit-test?

See also https://github.com/mozilla/pdf.js/wiki/Squashing-Commits

[yunfeizhu]

Thanks for the review. I've moved the regression coverage into the existing Reorganize Pages View > Save a pdf integration suite and squashed the changes into a single commit.

The new cases cover deleting one or three trailing pages, restoring the remaining page order, and checking the Save payload. Both fail without the fix and pass with it in Chrome and Firefox. The existing save test, generic build, and lint also pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (ee470d5) to head (808065a).
⚠️ Report is 84 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21972      +/-   ##
==========================================
+ Coverage   90.30%   90.31%   +0.01%     
==========================================
  Files         269      269              
  Lines       67537    67539       +2     
==========================================
+ Hits        60989    60999      +10     
+ Misses       6548     6540       -8     

Flag	Coverage Δ
browsertest	66.17% <50.00%> (+<0.01%)	⬆️
fonttest	8.92% <ø> (ø)
integrationtest	69.36% <100.00%> (+0.01%)	⬆️
unittest	59.50% <100.00%> (-0.01%)	⬇️
unittestcli	58.00% <100.00%> (+0.01%)	⬆️

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