Description

_plotly_utils/utils.py has a family of small pure string helpers used to
build the "did you mean" error messages for invalid property paths - splitting
a path, computing character positions, marking them, and suggesting the
closest valid key. Several of them had no direct test coverage.

This adds tests/test_core/test_utils/test_string_utils.py covering:

• split_multichar (split on any of several chars; empty char list is a
  no-op)
• split_string_positions (first-character index of each split piece)
• display_string_positions (single index, offset/length/char/trim
  options, and the all-indices form) - using the exact examples from the
  docstrings
• chomp_empty_strings (basic, leading-empty, no-op, the documented special
  cases []/['']/['','']/['','','',''], and reverse)
• levenshtein (known distances, zero, empty operand, symmetry)
• find_closest_string (nearest by edit distance, with lexicographic
  tie-break)
• _natural_sort_strings (numeric-aware ordering, and reverse)

No source changes.

Verification

pytest tests/test_core/test_utils/test_string_utils.py - 15 passed.
ruff check / ruff format clean.