feat: Simplify CASE WHEN true THEN expr to expr

[EeshanBembi]

Summary

Adds an optimization rule to simplify CASE expressions where the first condition is always true (literal true), reducing them to just the THEN expression.

This fixes the issue where expressions like:

CASE WHEN true THEN 1 ELSE x END

were not being simplified to just 1.

Changes

• Added new simplification rule in expr_simplifier.rs that detects CASE WHEN true THEN expr patterns
• Added comprehensive tests covering various scenarios including:
  • Simple literal values
  • Column references
  • Multiple WHEN clauses (only first is simplified)
  • Cases with and without ELSE clauses

Test Plan

• Added unit tests for the new simplification rule
• Verified existing CASE tests still pass
• Tested with the exact example from the issue
• Confirmed logical and physical plans now show simplified expressions

Before/After

Before:

logical_plan  | Projection: CASE WHEN Boolean(true) THEN Int64(1) ELSE CAST(foo.x AS Int64) END
physical_plan | ProjectionExec: expr=[CASE WHEN true THEN 1 ELSE CAST(x@0 AS Int64) END as ...]

After:

logical_plan  | Projection: Int64(1) AS CASE WHEN Boolean(true) THEN Int64(1) ELSE foo.x END
physical_plan | ProjectionExec: expr=[1 as CASE WHEN Boolean(true) THEN Int64(1) ELSE foo.x END]

Fixes apache#17448