fix(decimal): use minimal byte length for negative powers of two

[vishnuprakaz]

Closes #3522

Rationale for this change

bytes_required should return the minimum number of bytes for
a value, but it returns one byte too many for negatives like
-128 and -32768.

This matters because the decimal bucket transform hashes those
bytes. The extra byte changes the hash, so PyIceberg can put a
value in a different bucket than Spark/Java. For example,
Decimal("-1.28") goes to bucket 12 in PyIceberg but bucket 13
everywhere else.

The fix computes the length from (value + 1) for negative
values, which gives the true minimum and matches the Iceberg
spec.

Are these changes tested?

Yes. Added tests for the -128 / -32768 / -8388608 boundary cases
(where the old code was wrong) plus positive/negative controls.
Lint and the decimal/conversion/transform tests pass.

Are there any user-facing changes?

Yes. For decimal values like -1.28, the computed bucket changes
(e.g. 12 → 13) so it now matches other Iceberg engines.