fix: support parentheses for negative decimal formatting

[wangzhigang1999]

Which issue does this PR close?

• Closes Spark format_string ignores parentheses flag for negative decimals #23717.

Rationale for this change

Spark and Java format negative numeric values with parentheses when the ( flag is present. The decimal formatting path always emitted a minus sign because it ignored negative_in_parentheses, while the floating-point path already handled the flag. This made format_string inconsistent across numeric input types.

What changes are included in this PR?

• Add a closing suffix when a negative decimal uses parentheses formatting.
• Include the suffix when calculating width, left alignment, and zero padding.
• Add regression coverage for grouped negative decimals with and without an explicit width.

Are these changes tested?

Yes. The following checks pass locally:

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p datafusion-spark --lib
• The full workspace test command from the contributor guide with avro,json,backtrace,extended_tests,recursive_protection,parquet_encryption enabled

Are there any user-facing changes?

Yes. Spark-compatible formatting of negative decimal values now honors the parentheses flag. There are no public API or breaking changes.