Post review follow-up for preload_latest_diff_commit tests

What does this MR do and why?

This MR addresses post-review follow-up items from !203465 (comment 2759674617)

Follow-up items addressed:

1. Remove safe navigation operator (!203465 (comment 2759674617)):

   • Removed &. operator from commit.merge_request_commits_metadata&.commit_author and commit.merge_request_commits_metadata&.committer
   • Since we're creating metadata during setup for with_metadata tests, the safe navigation is unnecessary
   • This gives us confidence that the whole method chain is called without exceeding query limits

2. Split query limit specs for better isolation (!203465 (comment 2759674591)):

   • Separated the query limit expectations for with_metadata and without_metadata cases
   • with_metadata case expects max 7 queries
   • without_metadata case expects max 4 queries
   • This provides better clarity and isolation between the two scenarios

3. Improve test expectations (!203465 (comment 2759674564) and !203465 (comment 2759674572)):

   • Changed from expect { ... }.not_to raise_error to expect(...).to eq([expected_result])
   • Testing the end result automatically validates that no error was raised while being more explicit about expectations

References

• Original MR: !203465 (merged)
• Follow-up discussion: !203465 (comment 2759674617)

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