[6.x] Fix static cache invalidation rules for multisite with relative site URLs

[joshuablum]

This fixes static cache invalidation rules silently doing nothing on multisites whose site URLs are relative (e.g. /de/, /fr/).

Relative invalidation rules were prefixed with $site->url(), producing relative URLs like /de/*. Since there's no domain, the cacher falls back to getBaseUrl(), which is app.url plus the current site's path (https://example.com/de). Cached URLs are stored under the host only (https://example.com), so the lookup hits an empty bucket and nothing is invalidated.

Entries mask this because their own absoluteUrl() is always invalidated. Navigations, globals, forms, assets, etc. rely entirely on rules, so they appear broken.

Separately, absolute rules weren't tidied. A rule like env('APP_URL').'/de/*' with a trailing slash on APP_URL becomes https://example.com//de/*, which never matches either.

The fix

• Prefix relative rules with $site->absoluteUrl(), matching how entry URLs are already resolved.
• Run URL::tidy() over absolute rules too.
• Replace the nine copies of the rule-to-URL logic with a single resolveRuleUrls() helper.
• Preserve the final * when tidying wildcard rules so trailing-slash enforcement doesn’t break invalidation or background refresh.

[edalzell]

@joshuablum does this close this #3291 ?

[joshuablum]

@edalzell Maybe? Issue popped up through a support ticket. Will take a look tomorrow 👍