[expr] Placeholder inference for CASE statements

### Is your feature request related to a problem or challenge?

Placeholder inference should work for CASE expressions, i.e.

```sql
-- Simple CASE with placeholders
SELECT CASE department_id WHEN $1 THEN 'Match' ELSE 'No match' END FROM employees;

-- Searched CASE with placeholders
SELECT CASE WHEN salary > $1 THEN $2 ELSE $3 END FROM employees;

-- CASE with placeholder as base expression
SELECT CASE $1 WHEN 'Engineering' THEN 'Eng' ELSE 'Other' END;
```

### Describe the solution you'd like

`Expr::infer_placeholder_types` should infer the data types of placeholders for CASE expressions.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reading Expr::infer_placeholder_types and trace how it handles CASE expressions and placeholder types. Use the three SQL examples in the issue as coverage cases; done means placeholders in simple CASE, searched CASE, and CASE-base expressions receive inferred data types.

Written by the indexing model from the issue text.