Fix ada::url host_end one byte short of its documented position - #1244

ada::url::get_components().host_end is one byte short of the position the url_components documentation diagram describes (one past the host) in both branches of url-inl.h: the no-credentials branch subtracts one, and the credentials branch omits the '@' byte that host_start sits on. ada::url_aggregator reports the documented position on the same inputs, so the two implementations of the one contract disagree, and slicing href by [host_start, host_end) from ada::url truncates the host:

url             https://example.com/path            host_end=18  substr -> "example.co"
url_aggregator  https://example.com/path            host_end=19  substr -> "example.com"
url             https://user:[email protected]/path  host_end=28  substr -> "@example.co"
url_aggregator  https://user:[email protected]/path  host_end=29  substr -> "@example.com"

The computation was internally self-correcting - running_index added the missing byte back - which is why every other offset agrees between the two implementations and only the exposed host_end is wrong. The existing test slices by get_hostname().size() and never reads host_end, which is how it survived. This change fixes both branches, updates running_index accordingly (it is now already one past the host), and pins [host_start, host_end) to the hostname in url_components.cpp, which runs over the full WPT urltestdata corpus. The new assertion fails before the fix and passes after; the full suite (353 tests) is green with the change.

Per AI_USAGE_POLICY.md: this defect was found by an automated analysis loop built on Claude; the fix and test were reviewed by me before filing, I am accountable for them, and I am happy to answer questions during review. clang-format 22 has been applied to the touched files.