git-reader: Add safety check for records JSON

[leplatrem]

If extraneous files are stored in the Git repo, this would prevent treating them as record.

If we had had such a safety check, we would have avoided the v42 crash on timestamp files introduced in #1557 (see RMST-517)

  File "/app/.venv/lib/python3.14/site-packages/anyio/_backends/_asyncio.py", line 2706, in run_sync_in_worker_thread
    return await future
  File "/app/.venv/lib/python3.14/site-packages/anyio/_backends/_asyncio.py", line 1100, in run
    result = context.run(func, *args)
  File "/app/app.py", line 800, in collection_changeset
    timestamp, metadata, changes = git.get_collection_changeset(
  File "/app/app.py", line 276, in wrapper
    return func(*args, **kwargs)
  File "/app/app.py", line 388, in get_collection_changeset
    record.get("last_modified", 0) for record in records_by_id.values()
AttributeError: 'int' object has no attribute 'get'