CASE WHEN expression with execution context and DataFusion equivalenc…

[lukekim]

This pull request updates how CASE WHEN expressions are evaluated in both benchmarks and tests, switching from the older .evaluate() method to the newer apply + execute pattern using a session context. This ensures consistency with the main execution path and prepares the codebase for future enhancements. Additionally, a new integration test is added to verify equivalence between Vortex and DataFusion CASE WHEN evaluation.

Migration to apply+execute evaluation pattern:

• All benchmarks in case_when_bench.rs now use the apply + execute pattern with a shared VortexSession context, replacing direct calls to .evaluate() for more accurate benchmarking of real execution paths. [1] [2] [3] [4] [5]
• All tests in case_when.rs are refactored to use a helper function that evaluates expressions via apply + execute with a static session, replacing .evaluate() calls throughout. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Testing and correctness improvements:

• Added a new integration test to exprs.rs that checks the equivalence of CASE WHEN evaluation between DataFusion and Vortex, ensuring that the conversion and execution yield identical results.

These changes improve test coverage, future-proof the codebase, and ensure consistency between benchmarks, unit tests, and production execution.