TS: Validate required fields inside slice elements

[ryukzak]

Summary

• Add validateSliceFields runtime helper that checks required fields within matched slice elements
• Generate validateSliceFields calls for slices with required fields beyond match keys or constrained choices
• For BP profile: validates that valueQuantity is present in systolic/diastolic component stubs

Before:

profile.validate().errors
// ["at least one of effectiveDateTime, effectivePeriod is required"]

After:

profile.validate().errors
// [
//   "at least one of effectiveDateTime, effectivePeriod is required",
//   "component[SystolicBP].valueQuantity is required",
//   "component[DiastolicBP].valueQuantity is required",
// ]

Closes #123