Implement CQL define statements that use FHIRPath-based fluent functions to filter and transform retrieved FHIR resources and handle null safety to avoid null pointer evaluation failures

Verified steps

1. Declare the CQL library with using FHIR version 4.0.1 and include FHIRHelpers to enable automatic type conversion between FHIR primitives and CQL types; without FHIRHelpers, FHIR.string and System.String are incompatible and boolean comparisons on FHIR coded elements will not evaluate correctly
2. Use the exists() operator in CQL before accessing properties of a potentially empty list or null singleton to guard against null propagation; wrapping expressions in Coalesce() when a default value is acceptable rather than null propagation is preferable for metric calculations
3. Write retrieve expressions with code filters using value set membership rather than direct code comparisons wherever possible; use in keyword with a declared value set to leverage the engine's terminology expansion caching rather than calling ConceptIn() with inline code literals that are not cached
4. Use the where clause on list retrieve expressions to apply additional date or status filters that narrow the retrieved set; this communicates data requirements to the $data-requirements operation so that pre-fetching is scoped rather than loading all resources of that type
5. Test null safety by deliberately evaluating expressions against patients who have no resources of the retrieved type and confirm that the define returns null or false rather than a runtime error; CQL null propagation converts most null references to null silently but some function calls throw when passed null
6. Use the CQL debug mode on cqf-ruler by passing a debug extension parameter on the $cql call to receive a detailed evaluation trace showing the value of every define statement and intermediate expression, which reveals exactly where null propagation enters the expression tree