Refactor pkg/stringutil by feature and centralize fuzzy separator normalization

[Copilot]

pkg/stringutil/stringutil.go had accumulated unrelated helpers, making the package harder to navigate versus the existing one-file-per-feature pattern. This change reorganizes those helpers into focused files and removes duplicated inline separator normalization in fuzzy matching.

• String utility file decomposition

  • Moved whitespace helpers into whitespace.go:
    • NormalizeWhitespace
    • NormalizeLeadingWhitespace
  • Moved numeric/version parsing helpers into numbers.go:
    • IsPositiveInteger
    • ParseVersionValue
  • Kept general-purpose formatting/truncation helpers in the general file.

• Fuzzy match separator normalization

  • Extracted _ → - normalization into a small helper in fuzzy_match.go.
  • Replaced duplicated inline strings.ReplaceAll(..., "_", "-") calls for target/candidate normalization with the helper.

• Behavioral scope

  • No exported API signatures changed.
  • Refactor is package-internal organization plus local helper extraction for readability and consistency.

func normalizeFuzzySeparators(s string) string {
	return strings.ReplaceAll(s, "_", "-")
}

• Fixes [refactor] stringutil.go grab-bag + scattered separator normalization #50459