[ikkyu3]

Summary

Adds regression tests for host-parsing boundaries around :: in IPv6
literals — group count, per-group hex width, and lone (non-doubled)
leading/trailing colons — that aren't currently exercised by
tests/wpt/urltestdata.json's existing IPv6 cases.

Motivated by #1233, which rewrote this parser; the new tests pin down the
exact WHATWG grammar boundaries it (and the scalar fallback) must continue
to enforce, independent of which code path handles a given build.

What's covered

• :: must elide at least one group (all-8-groups-explicit + :: is invalid)
• a single elided zero-piece is not re-compressed to :: on serialization
  (only runs of ≥ 2 are, per spec)
• a group holds at most 4 hex digits
• uppercase hex digits are accepted and normalized to lowercase
• a lone leading or trailing colon is invalid even when a genuine ::
  appears elsewhere in the same host
• more than one :: is invalid
• the full-width, no-compression boundary (8 groups × 4 hex digits, the
  longest valid non-compressed address)

Test plan

• New tests pass for both ada::url and ada::url_aggregator
• Full basic_tests suite passes locally (207/207)
• clang-format clean (LLVM 22, project config)

[lemire]

Running tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.28%. Comparing base (9aa61f0) to head (071fc96).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1242      +/-   ##
==========================================
+ Coverage   63.25%   63.28%   +0.02%     
==========================================
  Files          39       39              
  Lines        7699     7699              
  Branches     3514     3514              
==========================================
+ Hits         4870     4872       +2     
  Misses        749      749              
+ Partials     2080     2078       -2     

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