feat(strict-mode): render nested matches as tree in fetchDetails()

[DavertMik]

Strict-mode failures list every matched element flat, so when one match is nested inside another the relationship is invisible.

This change derives nesting from absolute-XPath segment prefixes and renders child matches indented with an explicit parent label, keeping global numbering (numbers still map to grabWebElements()[i] / elementIndex). Flat output for non-nested matches is byte-identical to before.

Multiple elements (4) found for ".item" in strict mode.
  1. > //html/body/div[1]
     <div class="item">…
    2. (inside 1.) > //html/body/div[1]/div[1]
       <div class="item">…
    3. (inside 1.) > //html/body/div[1]/div[2]
       <div class="item">…
  4. > //html/body/footer
     <footer>…</footer>

• All logic in lib/helper/errors/MultipleElementsFound.js (pure splitXPath / isAncestorXPath / computeParents / computeDepths / formatTree helpers, exported for tests); no extra browser calls, no helper changes
• The (inside N.) label names the immediate parent's match number, so deeper nesting reads (inside 2.), etc.
• Failed lookups render as label-free roots without breaking ancestry; 10-item cap and footer unchanged
• Tests: new test/unit/multiple_elements_found_test.js (7 tests), ESLint clean