Element hiding rules (cosmetic filters) do not work with regex

For example (tested using latest Brave 1.66.113 on macOS and iPadOS):
`/example\.com/` - blocks the website
`example.com##.adClass` - hides the appropriate element
however,
`/example.com/##.adClass` - does not work (does nothing). It does work in uBlock Origin

The behaviour was consistent across multiple websites. My apologies if this is expected behaviour but I couldn't find any mention of this in the issues open or closed.

Start by reproducing the reported difference between `/example\.com/##.adClass` and the working blocking and plain cosmetic-filter examples in the Rust adblock engine. Trace how regex host patterns are parsed and applied to cosmetic filters, then add coverage showing the regex-host cosmetic rule hides the matching element without regressing the existing cases.

Written by the indexing model from the issue text.