test: add edge-case coverage tests for validation, strings, URL conversion, comparison, encoding, and caching

Cover previously untested branches including qualifier key/value length
limits, command injection detection in qualifiers/version/subpath,
C1 and Unicode dangerous character detection, URL-to-purl parsing for
multiple registries, wildcard pattern length rejection, cache eviction
in both the wildcard regex cache and the flyweight purl cache.

Author: jdalton