Extend `CastUtils.java` with two helpers for decimal precision adjustment
and use them from `Cast.changePrecision` (both the eval and codegen
implementations). The new helpers mutate the input `Decimal` in place
(matching the behavior of the existing inline codegen), so they're safe
to call on the temporary produced by `Decimal.fromString(...)` /
`Decimal.apply(...)` / decimal-arithmetic results.

Helpers added:
* `changePrecisionExact(Decimal, int, int, QueryContext)`: ANSI throw on
  overflow, preserves the per-call-site `QueryContext` so error messages
  keep their query-origin info.
* `changePrecisionOrNull(Decimal, int, int)`: non-ANSI, returns `null`
  on overflow (no `QueryContext` needed).

`Cast.scala` changes:
* `changePrecision` eval method dispatches on `nullOnOverflow` and
  delegates to the appropriate helper.
* `changePrecision` codegen method has three branches now: the existing
  `canNullSafeCast` fast path (unchanged), a `nullOnOverflow` branch
  (inline), and the ANSI throw branch which now emits a one-line
  `CastUtils.changePrecisionExact(...)` call instead of the 5-line
  `if/else` overflow block.

Part of SPARK-56908 (umbrella). The ANSI throw branch of
`Cast.changePrecision` is hit by every cast to decimal that may overflow
(very common in TPC-DS, where `cast(int as decimal(7,2))` is widespread).
Collapsing the 5-line inline body to one line shrinks the generated
Java source for those plans.

No. The compiled behavior is identical; only the emitted Java source
text changes.

```
build/sbt "catalyst/testOnly *CastSuite *CastWithAnsiOnSuite \
  *CastWithAnsiOffSuite *AnsiCastSuite *TryCastSuite *DecimalSuite \
  *ExpressionClassIdentitySuite"
```

337/337 pass.

Generated-by: Cursor 1.x

### What changes were proposed in this pull request?

Extend `CastUtils.java` with `stringToBooleanExact(UTF8String, QueryContext)`
and use it from `Cast.scala` for the ANSI `String -> Boolean` cast path
(both eval and codegen). The non-ANSI path keeps the inline
`if/else if/else evNull = true` form because it has no error to throw.

### Why are the changes needed?

Part of SPARK-56908 (umbrella). The ANSI String->Boolean cast emits an
8-line `if (isTrueString) … else if (isFalseString) … else throw` block
in codegen. This PR collapses it to a one-line `CastUtils
.stringToBooleanExact(...)` call.

### Does this PR introduce _any_ user-facing change?

No. The compiled behavior is identical; only the emitted Java source
text changes.

### How was this patch tested?

```
build/sbt "catalyst/testOnly *CastSuite *CastWithAnsiOnSuite \
  *AnsiCastSuite *TryCastSuite"
```

204/204 pass.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Cursor 1.x