fix(code-editor): allow extension on nested elements

[aleksandrkislitsyn]

Fixes HealthSamurai/sansara#8121.

Problem

The FHIR JSON editor flagged a valid extension on a nested element as an Unknown property, and did not offer it in autocomplete. Examples from the issue:

• QuestionnaireResponse.item[].extension
• Patient.contact[].extension

Both resources are valid and save fine — the editor's client-side validation was wrong.

Root cause

resolveElements (in fhir-autocomplete.ts) drives both the diagnostics and the autocomplete. It resolves the valid child elements for a path by walking the StructureDefinition differentials.

• For a complex datatype (e.g. contact.name → HumanName), it descends via collectAllElements(typeCode), which recurses into baseDefinition (Element) and therefore picks up the inherited id/extension.
• For a BackboneElement (e.g. QuestionnaireResponse.item), there is no named type to resolve — its children live inline in the parent resource's differential. That branch just repoints the path and returns the inline children, and never merges in the BackboneElement/Element base fields. A differential doesn't repeat inherited elements, so id/extension/modifierExtension were missing.

Result: extension on a nested backbone/complex element was neither recognized (→ false "Unknown property") nor suggested.

Fix

Append the universal element properties to resolveElements output when not already present:

• id, extension — on every element
• modifierExtension — additionally on BackboneElements and resources (tracked via an isBackbone flag), not on plain complex datatypes like HumanName

Because both the diagnostics and autocomplete consume resolveElements, this one change fixes both: no more false "Unknown property" on nested extensions, and autocomplete now suggests extension (and modifierExtension where valid) inside nested backbone elements. The if not already present guard makes it a no-op at the resource root and for complex types, where these fields are already present.

Verification

• Reproduced both reported cases in the running editor; the false diagnostics are gone after the fix.
• Autocomplete now offers extension inside QuestionnaireResponse.item / Patient.contact, and modifierExtension on backbone items but not inside a plain HumanName.
• A genuinely invalid key is still flagged (validation not weakened).
• biome check clean on the changed file; tsc -b no new errors.

🤖 Generated with Claude Code

[Aitem]

LGTM