feat: Implement %rowIndex environment variable

[piotrszul]

Summary

• Implement %rowIndex environment variable for ViewDefinition forEach and forEachOrNull iterations, as defined in the SQL on FHIR ViewDefinition spec.
• %rowIndex provides the 0-based index of the current element within the iterated collection. At the top level (no iteration), it evaluates to 0. Each nesting level maintains independent values.
• Uses Spark's indexed transform(array, (element, index) -> ...) to track element positions during unnesting, threading the index through ProjectionContext into FHIRPath evaluation as a supplied variable.

Key changes

File	Change
ColumnRepresentation	Added transformWithIndex() using Spark's indexed transform
SingleResourceEvaluator	Added withVariable() for creating evaluators with additional variables
ProjectionContext	Added rowIndex field, withRowIndex(), and %rowIndex injection in evalExpression()
UnnestingSelection	Rewrote evaluate() to use indexed transform, passing element index as %rowIndex

Test coverage (8 test cases in viewTests/rowindex.json)

• 3 spec examples: element position, nested iteration with independent indices, unionAll with inherited/independent indices
• Top-level %rowIndex defaults to 0
• forEach and forEachOrNull with %rowIndex
• forEachOrNull with empty collection (null row)
• Arithmetic expression (%rowIndex + 1)

Test plan

• All 8 new %rowIndex tests pass via FhirViewExtraTest
• All 18 existing view tests pass (no regressions)
• All 41 existing FhirViewExecutorTest + FhirViewBuilderUnitTest tests pass
• Pre-existing compliance test failures unchanged (2 failures unrelated to this change)

Closes #2560

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud