fix: do not overflow in unwrap_cast for decimals with a negative scale

[edubraqd]

Which issue does this PR close?

• Closes Comparing a decimal with a negative scale to an integer literal panics in unwrap_cast #24896.

Rationale for this change

try_cast_numeric_literal computes 10_i128.pow(scale as u32) for both the literal's and the target's decimal scale. A negative scale wraps to a huge exponent, so simplifying arrow_cast(1, 'Decimal128(10, -2)') = 100 panicked with "attempt to multiply with overflow" in the unwrap_cast simplifier.

What changes are included in this PR?

A negative scale means the decimal holds multiples of 10^-scale, so the integer rescaling this path relies on does not apply. try_cast_numeric_literal now returns None when either side has a negative scale, so the simplifier leaves the cast in place, the same way it already does for other casts it cannot express.

Are these changes tested?

Yes. test_try_cast_to_type_negative_scale_decimal covers a negative-scale target for Decimal32/Decimal64/Decimal128 and a negative-scale literal; each used to panic.

Are there any user-facing changes?

No panic. Such comparisons are no longer simplified by unwrap_cast; they were never simplified correctly before, since the process panicked.

[kosiew]

@edubraqd

When I ran this in datafusion-cli (this PR):

SELECT arrow_cast(1, 'Decimal128(10, -2)') = 100;

I got

DataFusion CLI v55.0.0
+--------------------------------------------------------------+
| arrow_cast(Int64(1),Utf8("Decimal128(10, -2)")) = Int64(100) |
+--------------------------------------------------------------+
| false                                                        |
+--------------------------------------------------------------+

Shouldn't it be true?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.63%. Comparing base (d02bd78) to head (62269ce).
⚠️ Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/expr-common/src/casts.rs	96.96%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24897   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files        1123     1123           
  Lines      409546   409579   +33     
  Branches   409546   409579   +33     
=======================================
+ Hits       334319   334350   +31     
- Misses      55591    55596    +5     
+ Partials    19636    19633    -3     

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[alamb]

Marking as draft as I think this PR is no longer waiting on feedback and I am trying to make it easier to find PRs in need of review. Please mark it as ready for review when it is ready for another look

[alamb]

Closing per #24893 (comment)