fix: normalized Windows drive letter must be exactly two code points

[spokodev]

is_normalized_windows_drive_letter accepted any input of length >= 2 beginning with <ASCII alpha>:. Per the URL Standard a normalized Windows drive letter is a Windows drive letter, which is two code points, so the check should require exactly two.

The loose length let shorten a url's path treat a longer first segment that merely starts with <alpha>: as a drive letter and skip popping it on ..:

ada::parse("file:c:x/..")    -> "file:///c:x/"    (expected "file:///")
ada::parse("file:u:p@h/..")  -> "file:///u:p@h/"  (expected "file:///")

A real drive letter is unaffected: file:c:/.. still yields file:///c:/.

The fix changes >= 2 to == 2. All three call sites (shorten_path and the base-path drive-letter check in parser.cpp) pass a single path segment that should match only an exact drive letter, so the tighter check is correct for each.

Verified against the full WHATWG URL web-platform-tests urltestdata (no regressions) and added a regression test.

[lemire]

running tests

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.78%. Comparing base (1245a1e) to head (1e7308d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
include/ada/checkers-inl.h	0.00%	0 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1156      +/-   ##
==========================================
- Coverage   59.80%   59.78%   -0.02%     
==========================================
  Files          37       37              
  Lines        6127     6127              
  Branches     2978     2978              
==========================================
- Hits         3664     3663       -1     
  Misses        618      618              
- Partials     1845     1846       +1     

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