fix: preserve custom dataset payload profile

[jamesgao-jpg]

Summary

• Forward custom performance-case keyword arguments to PerformanceCase, preserving the selected payload profile.
• Add a regression test covering a custom dataset with payload_profile=vector.

Scope

PR #834 introduces top-level payload profiles for performance cases; this focused follow-up fixes the custom-dataset constructor path identified during review.

The related audit covered registered PerformanceCase constructors affected by that new argument. No other forwarding gaps were found: standard cases inherit the base initializer, while the other custom constructors explicitly forward **kwargs.

Testing

• tests/test_models.py::test_custom_performance_case_config_applies_top_level_payload (red before the fix, green after)
• tests/test_models.py::test_performance_case_config_applies_top_level_payload_without_mutating_custom_case

[sre-ci-robot]

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