test: test: add null-byte escape test cases for '#' handling

[shayannab]

Problem

When a project lives in a directory containing # (e.g. /home/user/f#/webpack),
webpack-dev-server fails to resolve modules because # is treated as a URL fragment
separator.

Ref: webpack/webpack#16819

Root Cause

The bug is not in parseIdentifier. The \0# null-byte escape is the designed
mechanism to embed a literal # in a filesystem path. The actual fix belongs
upstream in the code that constructs request strings from real filesystem paths so,
it must escape # as \0# before passing to the resolver.

Changes

Added test cases to test/identifier.test.js documenting the \0# escape mechanism:

• \0# in path → unescaped to #, no fragment produced
• \0# dir + real fragment at end
• multiple \0# escapes + real fragment
• \0# with both query and fragment
• unescaped # still treated as fragment start (regression guard)
• Windows paths with \0# escape

No production code was changed.

Test Results

All 653 tests pass ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.41%. Comparing base (39d56d3) to head (7e16d32).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #473   +/-   ##
=======================================
  Coverage   93.41%   93.41%           
=======================================
  Files          49       49           
  Lines        2384     2384           
  Branches      717      717           
=======================================
  Hits         2227     2227           
  Misses        128      128           
  Partials       29       29           

Flag	Coverage Δ
integration	93.41% <ø> (ø)

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