ExpediaGroup / bull / #1406

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Fix/collection element field transformer null regression (#654)

* fix: prevent NPE when field transformer is applied to collection elements with flat field name mapping

When a CollectionPopulator transforms list elements (e.g. List<PersonSvc> -> List<Person>),
it calls transformer.transform(elem, targetClass) without a breadcrumb. In resolveEffectiveSource,
evalBreadcrumb("fieldName", null) returns just "fieldName", which could match a simple flat
field mapping (e.g. "name" -> "fullName"). This caused the lookup to incorrectly redirect to
the root source object instead of the current element, and getSourceFieldValue silently returned
null (swallowing the MissingFieldException because a transformer was defined), ultimately
passing null into the user's transformer function and triggering a NullPointerException.

Fix: guard the breadcrumb-based mapping lookup with isNotEmpty(breadcrumb) so root-source
redirection only occurs when there is an actual breadcrumb prefix in the path.

* test: add regression tests for collection element field transformer NPE

Add tests verifying that field transformers using flat field name transformation
correctly use the collection element as the source, not the root object.

New sample classes:
- FromFooWithFullName: source item with a fullName field
- FromFooContainingList: root source with id + List<FromFooWithFullName>
- ImmutableToFooWithSplitName: target item with name + surname fields
- ImmutableToFooContainingList: root target with id + List<ImmutableToFooWithSplitName>

New tests in MixedObjectTransformationTest:
- testFieldTransformerOnCollectionElementsUsesElementNotRootAsSource: verifies
  that withFieldMapping + withFieldTransformer on collection elements reads from
  the element, not the root (would have NPEd before the fix)
- testAllCollectionElementsAreIndependentlyTransformedFromTheirOwnSource: verifies
  each element is transformed from its own source independently

Also adds resetFieldsMapping() to Abstr... (continued)

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