Spark format_string ignores parentheses flag for negative decimals #23717

Describe the bug

The Spark-compatible format_string function does not honor the ( flag when formatting negative Decimal values. Instead of enclosing the magnitude in parentheses, the decimal formatting path always emits a minus sign.

The floating-point formatting path already handles this flag correctly, so decimal and floating-point values currently produce inconsistent results.

To Reproduce

Format a negative decimal using the grouping separator and parentheses flags:

SELECT format_string('%(,.2f', CAST(-1234.50 AS DECIMAL(10, 2)));

Current result:

-1,234.50

A fixed-width format such as %(,15.2f similarly produces a minus sign instead of parentheses.

Expected behavior

The result should match Spark/Java formatting semantics:

(1,234.50)

For %(,15.2f, the result should be padded to the requested width:

     (1,234.50)

Additional context

ConversionSpecifier::format_decimal reads the sign but does not apply negative_in_parentheses. Supporting the flag also requires accounting for the closing parenthesis when applying width, left alignment, and zero padding.