fix: increase ROUND decimal precision to prevent overflow truncation (#19926)

## Which issue does this PR close?

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- Closes #19921.

## Rationale for this change

`SELECT ROUND('999.9'::DECIMAL(4,1))` incorrectly returned `100.0`
instead of `1000`.

When rounding a decimal value causes a carry-over that increases the
number of digits (e.g., 999.9 → 1000.0), the result overflows the
original precision constraint. The overflow was silently truncated
during display, producing incorrect results.

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## What changes are included in this PR?

- Fixes ROUND on DECIMAL so carry-over doesn’t silently truncate/produce
wrong results.
- When decimal_places is a constant (including omitted/NULL), ROUND now
reduces the output scale to min(input_scale, max(decimal_places, 0))
(Spark/DuckDB-style), reclaiming
    precision for the integer part.
- When decimal_places is not a constant (e.g. a column/array), ROUND
keeps the original scale and may increase precision by 1 (capped); if
precision is already max and the rounded
value can’t fit, it returns an overflow error instead of a wrong value.
  - Adds/updates sqllogictests for these behaviors and edge cases.

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## Are these changes tested?

- Added new sqllogictest case for the specific bug scenario
- Updated 2 existing tests with new expected precision values
- All existing tests pass

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## Are there any user-facing changes?

| Aspect | Before | After |
  | --- | --- | --- |
  | ROUND('999.9'::DECIMAL(4,1)) | 100.0 (wrong) | 1000 (correct) |
| Return type (default/literal dp) | Decimal128(4, 1) | Decimal128(4, 0)
|

- Return type for DECIMAL inputs can change: with literal dp it
generally reduces scale; with non-literal dp it keeps scale and may
increase precision by 1.
- New error case: when precision is already max and dp is non-literal,
ROUND may now error on overflow rather than return a truncated/wrong
decimal.
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Co-authored-by: Martin Grigorov <martin-g@users.noreply.github.com>

Author: kumarUjjawal