Add unit tests for _resolve_external_table

[Copilot]

Adds test coverage for _resolve_external_table in tests/utils/test_dj_helper_fn.py, which previously had none despite handling critical external table updates (related to #1535).

Changes

• tests/utils/test_dj_helper_fn.py: 7 unit tests covering both location="analysis" and location="raw" branches, including:
  • Empty match → warning logged
  • Multiple matches → ValueError raised
  • Single match → update1 called with updated size and contents_hash
  • Mixed empty/non-empty analysis externals → only matching external updated

All tests use unittest.mock.patch to mock LabMember, AnalysisRegistry, schema, Path, and dj—no database or real file I/O required.

Checklist:

• N/A. If this PR should be accompanied by a release, I have updated the CITATION.cff
• N/A. If this PR edits table definitions, I have included an alter snippet for release notes.
• N/A. If this PR makes changes to position, I ran the relevant tests locally.
• N/A. If this PR makes user-facing changes, I have added/edited docs/notebooks to reflect the changes
• I have updated the CHANGELOG.md with PR number and description.

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