Feat: Allow pow with negative & non-integer exponent on decimals (#19369)

## Which issue does this PR close?
Closes #19348

## Rationale for this change
Previously, pow() on decimal types would error for negative exponents
and non-integer exponents with messages like:

- Arrow error: Arithmetic overflow: Unsupported exp value: -5
- Compute error: Cannot use non-integer exp
- This was a regression from when decimals were cast to float before
pow(). The efficient integer-based algorithm for computing power on
scaled integers cannot handle these cases.

## What changes are included in this PR?
- Modified pow_decimal_int to fallback to pow_decimal_float for negative
exponents
- Modified pow_decimal_float to use an efficient integer path for
non-negative integer exponents, otherwise fallback to f64 computation

Added pow_decimal_float_fallback function that:
- Converts the decimal to f64
- Computes powf(exp)
- Converts back to the original decimal type with proper scaling
- Added decimal_from_i128 helper to convert i128 results back to generic
decimal types (needed for Decimal256 support)
- Updated sqllogictests to expect success for negative/non-integer
exponents

## Are these changes tested?
Yes:

Unit tests for pow_decimal_float_fallback covering negative exponents,
fractional exponents, cube roots
Updated SQL logic tests in decimal.slt

## Are there any user-facing changes?
Yes. The following queries now work instead of returning errors:

```sql
-- Negative exponent
SELECT power(4::decimal(38, 5), -1);  -- Returns 0.25

-- Non-integer exponent  
SELECT power(2.5, 4.2);  -- Returns 46.9

-- Square root via power
SELECT power(4::decimal, 0.5);  -- Returns 2

Author: Yuvraj-cyborg