Decimal scalars panic on a stored value outside the precision #10749

What happened?

Two decimal operations panic when a stored value does not fit the precision of the dtype. DecimalArray::scalar_at (vortex-array/src/arrays/decimal/vtable/operations.rs:23) and the decimal min_max aggregate (vortex-array/src/aggregate_fn/fns/min_max/decimal.rs) build their results with Scalar::decimal, which calls vortex_expect on the precision check of Scalar::try_new (vortex-array/src/scalar/constructor.rs). DecimalArray construction does not check its values, and file reads trust the stored precision, so a file that holds such a value can make a read panic instead of returning an error.

Steps to reproduce

let array = DecimalArray::new(buffer![100i8], DecimalDType::new(2, 0), Validity::NonNullable)
    .into_array();

// Both calls panic.
array.execute_scalar(0, &mut ctx);
min_max(&array, &mut ctx, Default::default());

panicked at vortex-error/src/lib.rs:716:9:
unable to construct a decimal `Scalar`:
  Other error: decimal value decimal8(100) does not fit in precision of decimal(2,0)

Expected: an error from both calls, or a contract that rules out the value before it reaches them.

Environment

develop at 3974ee069c, run as a unit test in vortex-array.

Additional context

Scalar::decimal has other callers on read and compute paths, for example sum, mean, cast, fill_null, between, and the DecimalByteParts compare and between kernels. They have not been checked one by one. #10701 makes DecimalArray::try_new check every valid value, but file reads keep trusting the stored precision, so this stays reachable without a separate fix.