Return errors from decimal conversions

[osipovartem]

Summary

• make decimal_from_i128_with_scale and decimal_mantissa return Result instead of panicking on invalid scale or coefficients outside i128
• encode decimal manifest bytes directly from the U128 coefficient, preserving minimal two's-complement output without narrowing
• propagate conversion failures through DataFusion statistics and conservatively disable pruning for invalid decimal bounds
• replace the existing string-based constructor path with fixed-width integer/exponent operations

This mirrors the fix requested during review of JanKaul/iceberg-rust#405.

Performance

The successful constructor path no longer allocates intermediate strings or parses them back into D128. Decimal manifest serialization remains infallible and uses the fixed-width coefficient directly.

Verification

• cargo +1.95.0 test -p iceberg-rust-spec spec::decimal::tests
• cargo +1.95.0 test -p datafusion_iceberg converts_precision_38_decimal_bound_to_datafusion
• cargo +1.95.0 clippy -p iceberg-rust-spec -p datafusion_iceberg --all-targets -- -D warnings