Expose relational synthesis API on top-level dpsynth and strengthen MultiTableConfig eager validation.

[copybara-service]

Expose relational synthesis API on top-level dpsynth and strengthen MultiTableConfig eager validation.

• Public API Packaging:

  • Export essential multi-table relational synthesis constructs (MultiTableConfig, MultiTableMechanism, MultiDataGenerationResult, ForeignKeyRelation) directly on dpsynth and dpsynth.relational.
  • Update dependencies.

• Eager MultiTableConfig.post_init Validation:

  • Make foreign_keys a required argument with no default (= () removed).
  • Enforce naming constraints: reject table/column names containing '.' and reserved keywords ('group_size', 'slot_*').
  • Enforce schema invariants: reject empty table schemas and ensure all attributes are instances of NumericalAttribute, CategoricalAttribute, or OpenSetCategoricalAttribute.
  • Validate DAG hierarchy eagerly via topological_sort_hierarchy to immediately reject cycles, self-loops, multiple parents (in-degree > 1), and multi-root forests.
  • Enforce strict separation between relational key columns (PK/FK) and statistical domain schemas.
  • Validate custom initializer mappings (if provided) against domain tables and columns.
  • Enforce open interval hyperparameter bounds (0.0 < init_budget_fraction < 1.0).

• Testing:

  • Add unit tests for public module exports in synthesizer_test.py.
  • Add parameterized subtests covering all eager MultiTableConfig.__post_init__ validation failures.

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