[ClickHouse–Iceberg] Fix performance issue in decimal bounds processing for integer decimals

[hide42]

Context: ClickHouse–Iceberg integration (manifest decimal bounds)
Fix performance issue in decimal bounds processing for integer decimals

When processing decimal bounds in Iceberg manifest files, the code
had a severe performance issue for integer decimals (Decimal without
precision - whole numbers with scale = 0). The condition while (--scale)
when scale = 0 becomes while (-1) which is true, causing the loop to
iterate through all possible int32_t values (~4.3 billion iterations)
before stopping when scale wraps back to 0.

This resulted in approximately 3-second delay per integer decimal
value during bounds processing, significantly impacting performance.

Additionally, this could corrupt statistics because unscaled_value += scaler
would add an incorrect large value due to the massive scaler accumulated
over billions of iterations, leading to wrong bounds calculations.

Fix by adding a check if (scale) before the loop, ensuring the
scaling logic only runs for non-zero scale values (fractional numbers).
For integer decimals (scale = 0), no scaling is needed, so the scaler
remains 0, which is the correct behavior.

Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

...

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[CLAassistant]

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[kafka1991]

Upon confirmation this issue was also fixed in CH's 25.8.2(Included in PR #383)