[6.x] Keep .meta files out of the container listing cache

[edalzell]

AssetContainerContents::all() caches the raw recursive listing, .meta/*.yaml included. With cache_meta on that's one meta file per asset, so roughly half the cached payload is entries nothing reads through the listing. On one site that's 197k entries, an 80 MB serialized key, and an OOM on load.

The listing now rejects .meta paths (files and the .meta directories themselves) before caching. metaFilesIn() lists <folder>/.meta (or <folder> recursively) from the filesystem on demand instead of filtering the cache, and Asset::metaExists() checks the disk directly since the listing no longer knows about meta.

Tradeoffs: metaFiles() at the root recursively is now a full uncached disk walk per AssetContainerContents instance (only assets:meta:clean calls it today), and metaExists() is one disk exists() per syncOriginal() even when the CP already has the listing loaded. Left the .meta rejects in filteredFilesIn()/filteredDirectoriesIn() so an already-warm pre-upgrade cache still behaves.

#15609 is stacked on this and should merge after it.

Closes #15608

[ryanmitchell]

Love this, should make a big difference on our bigger sites too. Thanks Erin!

[edalzell]

Love this, should make a big difference on our bigger sites too. Thanks Erin!