fix: keep a false property subschema when a conditional branch adds a constraint

[thiamsantos]

Summary

Fixes a bug where a false property subschema was dropped when another allOf branch constrained the same property. The property then rendered as a normal field, and validation accepted a value the schema forbids.

Changes Made

mergeSchemaBranch merges each applied conditional branch into the base schema. When one branch set a property to false (forbidden) and a sibling branch added a constraint like { maximum: 12 }, the merge overwrote the false with the constraint object. A false subschema is unsatisfiable, so allOf semantics keep the property forbidden regardless of sibling constraints.

• Keep the false when merging property subschemas instead of overwriting it.
• Scope the check to property maps (properties / patternProperties) so a branch can still relax a boolean keyword such as additionalProperties: false.
• Fixes both paths: the field is now hidden, and a value for the forbidden property is now rejected.

Repro (before this change, createHeadlessForm with contract_duration_type: 'indefinite'):

allOf: [
  { if: { properties: { contract_duration_type: { const: 'fixed_term' } }, required: ['contract_duration_type'] },
    then: { required: ['months'] },
    else: { properties: { months: false } } },
  { if: true, then: { properties: { months: { maximum: 12 } } } },
]
// Before: `months` visible, and handleValidation({ months: 5 }) passed.
// After:  `months` hidden, and handleValidation({ months: 5 }) reports an error.

---

Note

Medium Risk
Changes core conditional schema merge logic used when building forms; scoped to property maps but affects visibility and validation for complex allOf schemas.

Overview
Fixes conditional allOf merging so a false property subschema (forbidden field) is not replaced when another branch adds constraints on the same key (e.g. { maximum: 12 }).

mergeSchemaBranch now tracks when it is merging inside properties / patternProperties and skips overwriting an existing false subschema in that context, matching JSON Schema allOf semantics. The guard is not applied to boolean keywords like additionalProperties, so branches can still relax those.

End-to-end behavior: forbidden fields stay hidden and validation rejects values that should not be allowed when a sibling branch only constrains the same property. New unit and visibility tests cover merge edge cases and the contract-duration / months scenario.

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