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 #17448

[alamb]

Thank you for the quick turnaround @EeshanBembi

I left some comments / suggestions

[alamb]

can we please add some negative tests too -- like CASE WHEN a THEN 1 ELSE 2 END?

[alamb]

can we please use is_true to follow the pattern in the rest of this method?

datafusion/datafusion/optimizer/src/simplify_expressions/utils.rs

Lines 193 to 198 in 4b9a468

	pub fn is_true(expr: &Expr) -> bool {
	match expr {
	Expr::Literal(ScalarValue::Boolean(Some(v)), _) => *v,
	_ => false,
	}
	}

[alamb]

I think it would be good to avoid this clone too,

How about something like this:

Suggested change

	Transformed::yes((*when_then_expr[0].1).clone())
	let (when, then_) = when_then_expr.swap_remove(0);
	Transformed::yes(*when)