fix: clear legacy cache variants for one URL

[Atroci]

Fixes #1086

Why

The admin-bar Delete Cache action clears the page's supercache directory but leaves legacy wp-cache files for the same URL. Query-string requests expose the gap because they are served by those legacy files, so a URL such as /some-post/?utm_source=test remains stale after deletion.

What Changed

• Keep nonce verification tied to the complete posted URI, then derive its normalized path for invalidation.
• Scan only the current $blog_cache_dir metadata and delete cache pairs whose URI has the same path, regardless of query string.
• Support current .php/JSON pairs and pre-2015 .html/serialized .meta pairs.
• Preserve child and sibling paths, malformed metadata, other multisite cache directories, and the separate whole-site deletion branch.

Verification

• Regression coverage removes bare, UTM, gclid, fbclid, functional-query, gzip-key, and historical HTML variants for one path.
• Coverage preserves child paths, sibling paths, malformed metadata, cache files outside the configured cache root, and another blog's cache directory.
• The real WordPress integration test posts a query-string path through capability and nonce checks, removes immediate HTML/gzip/wp-cache files, and preserves a child directory.
• PHPCS passes.
• Smoke suite: 85 tests, 134 assertions.
• Integration suite: 53 tests, 152 assertions in both single-site and multisite modes.

Release Notes

Fixed the Delete Cache action so it clears legacy wp-cache representations for the selected URL, including query-string variants, without recursively clearing child URLs.