Decouple PR review pagination tests from summary wording

[Copilot]

The pagination test was coupled to exact summary phrasing, allowing unrelated presentation changes to break data-path coverage.

• Pagination coverage
  • Validate both paginated threads and exact per-thread comment counts using full-detail output.
  • Confirm paginated comments (Comment 51) and thread-specific data are retained.
• Summary contract
  • Move wording assertions into a dedicated summary-format test.

assert example_output.count("@reviewer:") == 51
assert another_output.count("@reviewer:") == 1

• Fixes [autonomy-atomicity] PR review pagination test couples data-path verification to summary wording #1895