fix(arrow-cast): do not truncate integers when casting to Decimal32/64

[yongster]

Which issue does this PR close?

• Closes cast from integer to Decimal32/64 silently truncates values that do not fit #10706.

Rationale for this change

cast / cast_with_options from an integer array to Decimal32 or
Decimal64 can rewrite the value instead of rejecting it.

cast_integer_to_decimal first used AsPrimitive (as), which wraps when
the source integer does not fit the decimal native type. The precision check
then ran on the already-truncated value. If that wrapped value happened to
fit the requested precision, it was stored as if it were the original number.

Examples on current main:

• Int64(5_000_000_000) -> Decimal32(9, 0) (safe: true) becomes 705032704
• UInt32(4_000_000_000) -> Decimal32(9, 0) (safe: false) becomes -294967296
• UInt64::MAX -> Decimal64(18, 0) (safe: false) becomes -1

The same inputs cast to Decimal128 already return null / Err, because
i64 as i128 is lossless.

What changes are included in this PR?

• Convert the source integer through a range-checked i128 path
  (num_cast + DecimalCast) before applying scale and precision.
• safe: true writes null when the original value does not fit; safe: false
  returns Err and reports the original value.
• Add regression tests for Int64 -> Decimal32, UInt32 -> Decimal32, and
  UInt64::MAX -> Decimal64.

Are these changes tested?

Yes. I ran:

cargo test -p arrow-cast --lib -- cast
cargo clippy -p arrow-cast --all-targets --all-features -- -D warnings
cargo +stable fmt --all -- --check