Remove unused ETag from invalidation tags to prevent non-volatile tag Set leak

[jingu]

Problem

ResourceStorage::getTags() registers the ETag as an invalidation tag for every resource-state (roPool) save:

$tags = [$etag, ($this->uriTag)($ro->uri)];

However, no code path ever invalidates by ETag. All invalidation goes through the URI tag (deleteEtag() → invalidateTags([$uriTag])) or surrogate keys. HTTP 304 validation uses the separate etagPool (keyed by the ETag, not tagged by it), so it is unaffected.

Because the ETag is content-versioned (crc32 of the view), each content version creates a new, non-volatile tag Set (\0tags\0{etag}). Under a volatile-* eviction policy (required by RedisTagAwareAdapter), these Sets are never reclaimed and never read — a write-only memory leak that grows monotonically.

Production evidence

Measured on a production ElastiCache instance (full keyspace scan, read-only):

tag Set type	count	bytes
ETag (unused)	222,213 (61%)	75 MB
uriTag / surrogate (in use)	139,686	287 MB

The ETag Sets accounted for 61% of all tag Sets and were growing ~1.9k/hour, steadily consuming non-volatile memory that volatile-lru cannot evict.

Fix

Stop registering the ETag as an invalidation tag. The cache entry remains fully purgeable via its URI tag and surrogate keys, and HTTP 304 handling (the etagPool) is untouched.

Test

Adds testEtagIsNotRegisteredAsInvalidationTag: after saveValue(), invalidating by the ETag value must not purge the entry, while invalidating by the URI tag still does. Verified this test fails before the change (Failed asserting that null is not null) and passes after.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a4cd9e8) to head (e24c321).

Additional details and impacted files

@@             Coverage Diff             @@
##                 1.x      #180   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       247       247           
===========================================
  Files             53        53           
  Lines            749       748    -1     
===========================================
- Hits             749       748    -1     

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