fix: raise ValidationError for decimal precision outside 1-38 range

[PLTNGM]

Closes #3583

Rationale for this change

According to the Iceberg specification, decimal precision must be between 1 and 38. Previously, pyiceberg silently accepted invalid values (such as precision=39 or 0), which could lead to data corruption or crashes downstream when encoding/decoding fixed-byte decimals (matching the Java implementation's IllegalArgumentException).

This PR adds a defensive validation guard into DecimalType.__init__ to raise a ValidationError when the precision is out of bounds [1, 38].

Additionally, fixed an existing invalid type definition (DecimalType(100, 2)) inside tests/test_schema.py which was violating the specification bounds and causing the new validation guard to trip during the integration test suite.

Are these changes tested?

Yes, added dedicated unit tests in tests/test_types.py:

• test_decimal_precision_validation covers upper bounds (precision=39), lower bounds (precision=0, precision=-5), and ensures valid boundary values (precision=38) work perfectly.

Also updated existing test assertions in tests/test_schema.py to use a compliant precision value (38).

Are there any user-facing changes?

Yes, creating a DecimalType with an invalid precision will now explicitly raise a pyiceberg.exceptions.ValidationError instead of failing silently at runtime.