Support composite constraints in KG builder schemas

[adamnsch]

Description

Summary

• Add composite (multi-property) constraint support for UNIQUENESS and KEY
  constraint types
• ConstraintType gains a property_names: Tuple[str, ...] field; the old
  property_name: str field is deprecated but still accepted via a
  before-validator migration
• EXISTENCE constraints are validated to exactly one property (Neo4j
  doesn't support composite existence)
• Parquet metadata now includes a structured constraints list alongside
  per-column is_primary_key/is_unique booleans, preserving composite
  grouping for downstream consumers
• LLM extraction prompt updated to use property_names (list) format only
• All existing tests migrated from property_name to property_names;
  backward-compat test explicitly asserts the deprecation warning

Breaking changes

None. The old property_name field is still accepted in input (dicts,
JSON) and migrated automatically. Accessing constraint.property_name on
an instance emits a DeprecationWarning.

Testing

14 new unit tests cover: composite KEY/UNIQUENESS creation and validation,
EXISTENCE rejection of composite, backward-compat migration, property
validation, exclusivity checks, ParquetWriter metadata output with
composite constraints, LLM extraction round-trip for composite
KEY/UNIQUENESS, and extraction filtering (rejects composite EXISTENCE,
drops composites referencing nonexistent properties). Existing tests
updated to use property_names throughout.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Complexity: Medium

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate