fix(literals): return long bounds for decimal conversion

[abnobdoss]

Closes #3469

Rationale for this change

Decimal literals converted to LongType should use long bound sentinels when the value is outside the long range. The existing conversion returned integer sentinels, mismatching the requested target type.

This returns LongAboveMax and LongBelowMin for decimal-to-long overflow.

Relationship to Java

Java's DecimalLiteral.to(...) only handles DECIMAL and returns null for other target types (Literals.java#L497-L505), so there is no direct decimal-to-long branch to mirror. This keeps PyIceberg's long conversion consistent with its typed integer overflow handling. Java's generic valueless overflow sentinels are a broader semantic difference and are out of scope here.

Are these changes tested?

Yes. New literal tests cover decimal values above and below the LongType range.

Are there any user-facing changes?

Yes. Decimal literal conversion to LongType now reports overflow with the correct long sentinel type.

[ebyhr]

The change makes sense to me. I've verified that the added tests are valid regression tests that fail without this PR's change.