[P2] Script validation field == null does not match a field omitted on insert #1871

Problem

A script validation whose condition checks record.<field> == null does not fire on insert when that optional field is omitted entirely from the payload (the key is absent from the validation record context). It works on update and when the field is explicitly null.

Repro

• todo_task rule record.priority == "urgent" && record.due_date == null ...: creating {priority:'urgent'} with no due_date key → passes (should fail). PATCHing an existing task to urgent+null due → correctly blocked.
• Pre-existing compliance_assessment.failed_requires_remediation has the identical gap.
• expense_line receipt rule fires on insert only because receipt_attached has a defaultValue (always present).

Expected

On insert, normalize the validation record context so omitted optional fields read as null (so field == null matches).

Affected

@objectstack/objectql (validation context build).

Found while runtime-testing objectstack-ai/templates (all 9 templates via objectstack dev all) on @objectstack 9.5.1. Build + typecheck pass; these only surface at runtime.