test(fhir): add regression tests for empty and dangling-reference Bundle edges

[axelray-dev]

Closes #547.

Lock in two behaviors of to_bundle() that were correctly implemented but not explicitly tested:

• Empty input: returns a well-formed Bundle with empty entry list for transaction, collection, and batch bundle types.
• Dangling references: references to resources absent from the Bundle are preserved verbatim (not rewritten). Tests cover single, mixed internal/external, and multiple dangling references on one resource.

Test-only change. No source code modifications. All 18 tests pass (6 new + 12 existing).

pytest tests/unit/clinical/test_fhir_bundle_edge_cases.py -v
pytest tests/unit/clinical/test_fhir_bundle.py -v

[maziyarpanahi]

Thank you @axelray-dev. I reviewed this against #547 / OM-362 and added one small maintainer follow-up commit: test: format FHIR bundle edge cases.

What I changed:

• removed an unused import in the new edge-case test file;
• applied the repository Ruff formatter to wrap the long dangling-reference assertion.

The test coverage itself matches the issue: empty resource lists produce well-formed empty Bundles for the requested bundle types, dangling references are preserved verbatim, and mixed internal/dangling references keep the existing rewrite behavior for resources present in the Bundle.

Verification on the current PR checkout:

• PYTHONPATH=/private/tmp/openmed-pr-556 /Users/maziyar/Developer/openmed/.venv/bin/python -m pytest tests/unit/clinical/test_fhir_bundle_edge_cases.py tests/unit/clinical/test_fhir_bundle.py -q -> 18 passed
• /Users/maziyar/Developer/openmed/.venv/bin/ruff check tests/unit/clinical/test_fhir_bundle_edge_cases.py tests/unit/clinical/test_fhir_bundle.py -> passed
• /Users/maziyar/Developer/openmed/.venv/bin/ruff format --check tests/unit/clinical/test_fhir_bundle_edge_cases.py tests/unit/clinical/test_fhir_bundle.py -> passed

I also copied the labels from #547 onto the PR. The branch is mergeable with no conflicts; GitHub has not attached hosted checks to the new head commit yet, so I verified the touched behavior locally.