Allow whitespace after leading numeric signs and parentheses

[tannergooding]

Summary

• Accept whitespace after a permitted leading sign or opening parenthesis when NumberStyles.AllowLeadingWhite is set, matching the corresponding trailing-whitespace behavior.
• Keep ordinary integer parsing on its fast path and retry the shared parser for whitespace after a sign; align hexadecimal floating-point parsing and update AllowLeadingWhite documentation.
• Add regression coverage for numeric types, custom signs, parentheses, UTF-8, and partial parsing with consumed lengths. Without AllowLeadingWhite, the same inputs remain invalid.

Validation

• build.cmd clr+libs -rc checked -lc Release -- succeeded with no warnings or errors.
• System.Runtime numeric-type tests: 36,311 total, 0 failed, 71 skipped (platform conditions).
• System.Runtime.Numerics tests: 8,635 total, 0 failed.
• Eleven Int32 regression cases failed on unfixed upstream and passed with this fix.

Exploratory local timings found no consistent common-case JIT regression. ReadyToRun int.Parse with leading whitespace measured roughly 5.9 ns before vs. 6.4 ns after, but the difference did not persist in JIT-only runs and is sensitive to code layout. BenchmarkDotNet did not recognize the current .NET 11 runtime.

Resolves #101873

Note

This PR description was drafted by GitHub Copilot.

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