Query: prevent invalidated query caches from accumulating.

[JJJ]

Invalidating a query currently creates another persistent cache key. This maintenance fix stores the generation with the cached results and replaces stale results at a stable key, preventing successive invalidations of the same query from accumulating entries.

Remove unset-value sentinels from the query hash, following Robin Cornett's correction in #185, so equivalent query instances use the same key. The generation is captured before reading the database and checked when reading cached results.

This changes only two methods in src/Database/Query.php, with no new API, dependencies, or minimum runtime requirements. Existing orphaned cache entries are not removed by this fix. Distinct queries still have distinct cache entries; this does not introduce expiration.

Validation was performed outside the maintenance branch, with no test infrastructure added to the release:

• WordPress 6.7.7, PHP 8.2, MariaDB 10.2, and Redis 7: 14 tests, 60 assertions passed using a test-only Redis adapter.
• Default WordPress object cache: 14 tests, 56 assertions, one expected Redis-only skip.
• The initial regression suite failed eight tests against unpatched 2.0.2.
• PHP 7.4 syntax and git diff --check passed.
• Claude reviewed the patch and found no blocking defects; the additional suggested regression cases passed.

Props @hellofromahmed (Ahmed Saeed) for reporting the persistent-cache growth, and @robincornett (Robin Cornett) for the original unset-value cache-key correction.

See #253, #184, #185. This PR addresses the 2.0.x maintenance line; #253 stays open for the 3.x up-port.

[Copilot]

🟢 Approval recommended

The change is localized to query caching, aligns with the stated goal of preventing orphaned cache-key growth, and does not introduce new API surface.

Pull request overview

This PR adjusts src/Database/Query.php’s query-result caching so repeated invalidations do not generate accumulating, permanently-stored cache keys by switching to a stable cache key and embedding cache-generation (last_changed) in the cached payload.

Changes:

• Store last_changed alongside cached query results and ignore cached results when generations don’t match.
• Replace the query-result cache write from cache_add() to cache_set() to overwrite stale results under a stable key.
• Remove per-instance “unset-value” sentinel defaults from the cache-key hash so equivalent query instances share a key.

File summaries

File	Description
src/Database/Query.php	Makes query-result cache keys stable across invalidations and across equivalent query instances by embedding generation in the value and stripping sentinel defaults from the hashed slice.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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