fix factorial(21) should not overflow when PostgreSQL returns a numeric answer

[xiedeyantu]

Which issue does this PR close?

• Closes PostgreSQL compatibility: factorial(21) should not overflow when PostgreSQL returns a numeric answer #22259.

Rationale for this change

factorial was previously implemented using an Int64 lookup table, which caused factorial(21) to fail with an overflow error. This differs from PostgreSQL, which returns an exact numeric result for this input.

This change improves PostgreSQL compatibility by allowing factorial to return exact results beyond the i64 boundary, while still preserving overflow behavior once the supported decimal range is exceeded.

What changes are included in this PR?

• Change factorial to return Decimal256(76, 0) instead of Int64.
• Preserve the existing lookup-table fast path for values up to 20!.
• Extend factorial computation beyond the lookup-table range using checked i256 multiplication.
• Keep overflow behavior for inputs whose factorial exceeds the supported Decimal256 range.
• Add and update sqllogictest coverage for the new result and return type.

Are these changes tested?

Yes.

This PR updates sqllogictest coverage for factorial, including:

• exact result validation for factorial(21)
• return type validation for the new decimal result
• updated expectations for existing scalar and columnar factorial queries

Validated with:

cargo test -p datafusion-sqllogictest --test sqllogictests scalar

Are there any user-facing changes?

Yes.

factorial(21) now returns 51090942171709440000 instead of raising an overflow error, and the return type of factorial is now Decimal256(76, 0) rather than Int64.