fix(add_book): use _data.pop to strip infobase metadata from matched author

[TechGenius-Karan]

Closes #13016

**Fix**

The metadata cleanup loop in author_import_record_to_author() was broken in two ways:

1. existing.k accessed a literal attribute named "k" (always Nothing/falsy) instead of the loop variable k,
   so the condition was never True.
2. Even with the condition corrected, del existing[k] raises AttributeError because Infogami Thing objects
   have no __delitem__.

Fix: use existing._data.pop(k, None) to remove the metadata keys (last_modified, id, revision, created)
directly from the underlying data dict, which is the only supported mutation path on a Thing.

Technical

• Thing.__setitem__ delegates to self._getdata()[key], but there is no __delitem__ — mutation must go through
  _data directly.
• The mock_site.save() helper automatically appends revision, last_modified, and created to every saved
  document, so tests that return a matched author exercised this code path in CI.
• No behaviour changes to the matching logic itself — only the post-match metadata strip is fixed.

Testing

1. Run the affected test file:
   pytest openlibrary/catalog/add_book/tests/test_load_book.py -v
   All 35 tests should pass, including the new test_infobase_metadata_keys_stripped_from_existing_author which
   directly verifies that last_modified, revision, and created are absent from the returned author when a match
   is found.

2. Before this fix, 7 tests in that file failed with AttributeError: __delitem__ whenever find_entity() returned
   a match. All now pass.

Screenshot

N/A — no UI changes.

Stakeholders

[openlibrary-bot]

Thank you @TechGenius-Karan for this PR!

🤖 Copilot has been assigned for an initial review.

The linked issue hasn't been triaged yet — triage happens on Mondays and Fridays. There are currently 99 open non-draft PRs in the review queue.

PR triage checklist (maintainers)

• PR description — not empty; explains what the change does and how to verify it
• References an issue — PR body contains Closes #13016
  • Linked issue is triaged — has a Priority: * label (not just Needs: Triage)
  • Linked issue is assigned — has at least one assignee
• Commit history clean — no WIP/fixup/conflict noise; commit messages are meaningful
• CI passing — pre-commit.ci still running at time of comment
• Test cases present — 22 lines of new tests added in test_load_book.py ✓
• Proof of testing — PR body describes pytest openlibrary/catalog/add_book/tests/test_load_book.py and expected outcomes ✓

Note

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