AddDiffComment.add_comment iterated diffset.get_files(), which only yields

the first page of results (25 files by default). Any file beyond the first page

could not be matched, so rbt review add-diff-comment -f failed with

"Could not find a file ..." on diffs containing more than 25 files.
This changes the loop to iterate diffset.get_files().all_items, which follows

the next link and walks every page of the file list.
Regression tests are added in rbtools/commands/tests/test_review.py that build

a real two-page FileDiffListResource via URLMapTransport: one test posts to

a file on the second page (this fails without the fix, raising the

"Could not find a file" CommandError), and one covers a file on the first page

to guard the normal behavior.
Co-authored with Claude Code.

pytest rbtools/commands/tests/test_review.py — 2 passed.
Verified the second-page regression test fails without the fix (raising

CommandError: Could not find a file ...) and passes with it.