fix: support decimal datum conversion by precision

[abnobdoss]

Which issue does this PR close?

• Closes #.

What changes are included in this PR?

This fixes decimal datum precision handling in two places.

First, Datum::to() can now convert between decimal types with the same scale and different precision, as long as the value fits the target precision. This is needed when a decimal literal is created before the target column precision is known.

Second, this fixes a correctness issue in Datum::decimal_with_precision. It previously checked whether the mantissa fit the storage byte width for the requested precision, but that could accept values that exceeded the
declared decimal precision. It now checks the mantissa’s decimal digit count directly. For example, 0.42 has mantissa 42, so it does not fit decimal(1, 2).

Scale conversion remains out of scope. Decimal conversions with different source and target scales still return DataInvalid.

Are these changes tested?

Yes. This adds unit coverage for decimal precision calculation and decimal datum conversion.

Validation run:

cargo fmt --check
cargo test -p iceberg test_decimal_precision
cargo test -p iceberg test_datum_to_decimal_
cargo test -p iceberg test_datum_decimal_with_precision_
cargo clippy -p iceberg --tests -- -D warnings

Was generative AI tooling used to co-author this PR?

Yes.