Resolve Active Record models by their configured table name

[nertzy]

FixtureBuilder currently infers a model class from each table name with table_name.classify.constantize. This assumes the class name and table name follow Active Record conventions.

Models that use self.table_name = are not recognized when the configured table name cannot be constantized. FixtureBuilder then uses its model-less SQL extraction path instead of the model-backed path. This bypasses model-specific behavior such as custom Active Record type serialization.

Model discovery should honor each Active Record model's configured table_name while preserving the existing fallback for tables that genuinely have no model.

Acceptance criteria

• A model whose configured table name does not correspond to its class name is extracted through the model-backed path.
• Conventionally named models continue to work.
• Tables without an Active Record model continue to use the raw SQL fallback.
• Regression coverage distinguishes model-backed extraction from raw SQL extraction.