fix: scientific notation is incorrectly parsed in general numeric sort

[Kaua-Klassmann]

Description

This PR fixes the general numeric sort to parse correctly scientific notations with sign

Changes

• Change "peek_nth(2)" to "peek_nth(1)" on get_leading_gen
• Add "exponents-positive-general" test to validate general numeric sort

Fixes #10317

[codspeed-hq]

Merging this PR will degrade performance by 7.47%

❌ 1 regressed benchmark
✅ 141 untouched benchmarks
⏩ 180 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	sort_general_numeric[200000]	730.4 ms	789.3 ms	-7.47%

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_{Comparing Kaua-Klassmann:fix_scientific_notation_is_incorrectly_parsed_in_general_sort (307305f) with main (d2bd2d3)}

Footnotes

1. 180 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)