fix: Consolidate string-to-decimal parsing into a single code path

[neilconway]

Which issue does this PR close?

• Closes parse_decimal numeric overflow on long inputs #10787, closes parse_decimal panics or wrong result value on some valid inputs with negative exponents #10788, closes parse_decimal rejects 0e0 #10789, closes parse_decimal allows invalid inputs e5 and - #10790, closes parse_decimal ignores negative scales #10791, closes can_cast_types reports string → negative-scale decimal as supported, but cast always errors #10792, closes parse_decimal rejects input with leading/trailing whitespace #10793, closes variant_get converts strings to decimals without checking the target precision #10794

Rationale for this change

arrow-cast had two different string-to-decimal parsers: parse_string_to_decimal_native, used by cast, and parse_decimal, used by the CSV and JSON readers. Aside from redundancy, these code paths behaved differently (e.g., truncating vs rounding for digits beyond the target type's scale, whitespace trimming, support for e-notation, etc.), so decimal conversion behaved differently depending on how the decimal value arrived into arrow-rs.

This PR replaces these parsers with a single unified parser; parse_decimal(s, precision_scale) is now the public entry point and parse_string_to_decimal_native is a thin, deprecated wrapper over it. The new parser is based on the one-pass, u64-chunked parser in #10668, extended with support for e-notation and negative scales. This fixes a lot of bugs and ensures consistent behavior, but it does result in some behavior changes and a small performance regression for the CSV/JSON path; more details below.

Bugs fixed: (all in the JSON/CSV path)

• divide-by-zero panic or wrong values for some inputs in exponent notation (parse_decimal panics or wrong result value on some valid inputs with negative exponents #10788, Parsing Decimal With Negative Exponent And Zero Scale Can Overflow #5762)
• overflow on inputs with >= 256 digits or long exponents (parse_decimal numeric overflow on long inputs #10787)
• negative scales were ignored (parse_decimal ignores negative scales #10791)
• 0e0/-0e0 were incorrectly rejected (parse_decimal rejects 0e0 #10789), while e5 and -. were incorrectly accepted (parse_decimal allows invalid inputs e5 and - #10790)

Behaviour changes:

• CSV and JSON readers now round half away from zero instead of truncating digits beyond the scale (Parsing decimals truncates extra digits, but casting them rounds extra digits #9410, Parsing zero scale numbers truncates instead of rounding the number #9422, add rounding logic and scale zero fix in parse_decimal to match parse_string_to_decimal_native behavior #7355)
• cast from strings accepts exponent notation (Decimal enhancements in arrow-cast #5068) and negative scales, which can_cast_types already advertised (can_cast_types reports string → negative-scale decimal as supported, but cast always errors #10792), and validates the target precision and scale before parsing any values
• CSV and JSON readers now trim whitespace (parse_decimal rejects input with leading/trailing whitespace #10793). Only ASCII whitespace characters are trimmed, which matches the behavior of the CSV float/int parsers; previously, the cast path trimmed Unicode whitespace as well, but it will no longer do so.
• parse errors use ArrowError::ParseError with unified messages
• variant_get validates the target precision for string inputs (variant_get converts strings to decimals without checking the target precision #10794)

Performance:

• cast string-to-decimal: ~unchanged. The cast path already used the fast single-pass parser from perf(arrow-cast): optimize parsing of decimals from strings #10668; the unified parser benchmarks within Criterion noise (~4%) of it.

• CSV/JSON reader path: short inputs cost 1-2 ns more per value, while long Decimal256 inputs are ~35% faster. End-to-end, CSV reads of decimal columns are ~8% slower. I suspect there is room for further optimization here (which will now benefit both code paths!) to reach or exceed the previous performance, but I'd like to land the unified parser first before we tackle further optimizations.

What changes are included in this PR?

See above.

Are these changes tested?

New tests added to cover rounding, exponents, negative scale, whitespace, long inputs and the four widths, a seeded differential test checks 20k random inputs against a BigInt reference (num-bigint was added as a dev-dependency), and the CSV/JSON readers gain end-to-end tests for the new behavior and bugfixes listed above.

Are there any user-facing changes?

Yes; a deprecated public API, and user-visible behavioral changes in decimal parsing.

AI usage

Iterated primarily with CC Fable 5; code reviewed by Codex GPT 5.6. I read, understand, and revised the resulting code.