Advanced field types: unit-testable validation + VI/XCUI for interaction/a11y

[schatt]

Summary

Follow-up from #382 Slice 4 (AdvancedFieldTypesTests honesty pass).

Unit tests can observe stored configuration (allowedTypes, maxFileSize, suggestions, field, formState round-trips) and hostability. They cannot honestly claim:

1. Rich text editing mode toggle (private @State isEditing) without VI/XCUI
2. Suggestion selection UI (needs hosted interaction or extracted callbacks)
3. Drag-and-drop / browse file picker (selectFiles / handleDrop are placeholders)
4. Invalid type / max-size enforcement — product does not yet reject; no decision logic to unit-test
5. Accessibility labels/hints on the view tree — needs ViewInspector or XCUI on lanes that link VI

Acceptance

• Extract or implement pure validation for file type/size; unit-test it
• VI or XCUI coverage for editing mode, suggestion pick, upload area a11y identifiers
• Remove placeholder comments once behavior exists

Related

• Eliminate tautological Bool(true)/Bool(false) test expects #382 (tautological expects elimination)
• Layer priority: unit → hosted VI → XCUI