normalize typed DEFAULT_VALUE after JSON reload

[dpol1]

Purpose of the PR

• fix [Bug] Preserve typed DEFAULT_VALUE and struct userdata after JSON reload #3028

Normalizes PropertyKey default values to their declared data type upon retrieval. Previously, values stored in userdata could lose their original type during serialization and deserialization (for example, a DATE default reloaded as a String), leading to type mismatches when applying defaults.

The defaultValue() method in both hugegraph-core and hugegraph-struct now converts deserialized representations back to their expected runtime types using the existing type-aware converters based on PropertyKey.dataType(). This ensures that typed defaults (such as DATE) remain correctly typed after a JSON round-trip and when applied to vertices and edges.

This change is verified with tests covering schema parsing, vertex property assignment, and both binary and text serializers.

Main Changes

• Core PropertyKey.defaultValue() normalization

  • In hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/schema/PropertyKey.java, defaultValue() now:
    • reads Userdata.DEFAULT_VALUE from userdata,
    • if non-null, normalizes it via validValue(value), which routes through the existing type-aware converters (convValue / convSingleValue → dataType().valueToDate/Number/UUID/Blob).
  • This ensures that defaults reloaded from JSON as String (for typed keys such as DATE) are converted back to their runtime type before being used by consumers like HugeElement.updateToDefaultValueIfNone().

• Struct PropertyKey.defaultValue() mirror

  • Applies the same normalization logic to hugegraph-struct/src/main/java/org/apache/hugegraph/struct/schema/PropertyKey.java so that struct PropertyKey behaves consistently with core:
    • defaultValue() also normalizes the stored ~default_value according to the DataType.
  • Adds a focused unit test PropertyKeyTest.testDefaultValueNormalizedToDate() in hugegraph-struct that constructs a DATE property key with a String default and asserts that defaultValue() returns a Date.

• End-to-end vertex default application

  • Adds testAddVertexWithDateDefaultValue() to VertexCoreTest:
    • defines a DATE property key with Userdata.DEFAULT_VALUE set to a Date,
    • creates a vertex without explicitly setting that property,
    • commits, reloads the vertex from the backend (forcing schema reload),
    • asserts that the applied property is a Date with the expected value, not a String.
  • This reproduces and locks in the user-facing symptom from [Bug] Preserve typed DEFAULT_VALUE and struct userdata after JSON reload #3028 through HugeElement.updateToDefaultValueIfNone().

• Serializer round-trip for DEFAULT_VALUE

  • Extends TextSerializerTest and BinarySerializerTest with:
    • testPropertyKeyDefaultValueRoundTripsAsDate():
      • serializes a DATE PropertyKey whose DEFAULT_VALUE is a Date,
      • deserializes it back,
      • asserts that defaultValue() on the reloaded key returns a Date equal to the original.
  • Ensures that both text and binary serializers preserve the typed default after a full round-trip.

• Schema parsing normalization (core)

  • Extends SchemaElementTest with:
    • testPropertyKeyFromMapNormalizesDateDefaultValue():
      • builds a PropertyKey from a Map with DATA_TYPE=DATE and ~default_value as a formatted date String,
      • asserts that defaultValue() returns a Date.
    • testPropertyKeyFromMapNormalizesDateSetDefaultValue():
      • same as above but for Cardinality.SET, with ~default_value as a collection of date strings,
      • asserts that defaultValue() returns a Collection whose elements are all Date instances.
  • This validates both the scalar and collection branches of validValue() for DATE defaults.

Verifying these changes

• Need tests and can be verified as follows:
  • hugegraph-core:
    • VertexCoreTest.testAddVertexWithDateDefaultValue()
    • SchemaElementTest.testPropertyKeyFromMapNormalizesDateDefaultValue()
    • SchemaElementTest.testPropertyKeyFromMapNormalizesDateSetDefaultValue()
  • Serializers:
    • TextSerializerTest.testPropertyKeyDefaultValueRoundTripsAsDate()
    • BinarySerializerTest.testPropertyKeyDefaultValueRoundTripsAsDate()
  • hugegraph-struct:
    • PropertyKeyTest.testDefaultValueNormalizedToDate()

All existing tests remain green.

Does this PR potentially affect the following parts?

• Dependencies
• Modify configurations
• The public API
• Other affects (typed here)
  • Tightens the behavior of PropertyKey.defaultValue() for malformed or type-incompatible defaults: such values are now validated and may trigger conversion errors instead of leaking unexpected raw String values at read time.
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need