TypeSchema: Replace inline FHIRValue casts with proper FHIRCoding import

[ryukzak]

• Import FHIRCoding instead of using inline import("@atomic-ehr/fhirschema").FHIRValue casts
• Type codingValues as FHIRCoding[] (more precise than FHIRValue) using flatMap to safely filter undefined matches
• Remove as unknown as FHIRValue double cast — { coding: FHIRCoding[] } satisfies FHIRCodeableConcept which is a FHIRValue union member
• Remove redundant ternary that wrapped single-element arrays identically to multi-element