Refactor z-scoring tests to fix shapes and enable MNLE support

[satwiksps]

Refactor z-scoring tests to fix shapes and enable MNLE support

Description

This PR refactors test_z_scoring_structured in sbi/tests/sbiutils_test.py to address Issue #1559. It resolves variable naming confusion, removes code duplication, fixes dimensionality bugs, and enables testing for the mnle model which was previously skipped.

Changes

• Variable Renaming: Renamed builder to build_fn and net to estimator to clearly distinguish between the factory function and the instantiated network.
• Deduplication: Consolidated separate loops for different builders into a single parameterized test logic with a shared model list.
• MNLE Support: Added logic to generate mixed data (continuous + discrete) specifically for mnle, satisfying its input requirements and preventing crashes.
• Dimensionality Fixes:
  • Set num_dim = 3 for both theta and x. This ensures mnle has >1 continuous dimension (preventing NaN in std calculations) and allows transform_to_unconstrained to work via implicit bound checking without passing an explicit prior argument.
• Shape Correction: Added .unsqueeze(0) to flow inputs to satisfy the (sample, batch, dim) shape requirement, fixing AssertionError failures.
• Argument Safety: Removed the explicit prior kwarg from flow builders to prevent TypeError in Zuko flows, while ensuring bounds checks still pass via the x_dist fallback.

Related Issue

• Closes #1559

[codecov[bot]]

Codecov Report

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Additional details and impacted files

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