fix: DiskCache served a cache miss for entries that were still fresh

[jkmassel]

Fixes a bug where DiskCache.read(_:forKey:notOlderThan:) returned a cache miss for every entry inside the requested window, and a hit only for entries that had already aged past it.

Found while sweeping for the URL.path() shape from #26005. Unrelated to that bug, so it's on its own.

Summary

• The freshness comparison was inverted: it discarded entries that were still fresh and served entries that were stale.
• The same block resolved the file through a percent-encoded path string while the existing-file check two lines above used the URL, so the two disagreed for any key needing encoding.
• Both are latent. No caller passes notOlderThan: today, and every cache key in the app is an ASCII slug.

Root Cause

Modules/Sources/WordPressCore/DiskCache.swift

if creationDate.addingTimeInterval(interval) > Date.now {
    return nil
}

creationDate + interval is when the entry expires. The condition holds while that moment is still in the future — that is, while the entry is valid — and the branch returns nil. The comparison needs to be <.

The second issue is in the line above it:

guard FileManager.default.fileExists(at: path) else { ... }   // URL, decoded  ✅
let attributes = try FileManager.default.attributesOfItem(atPath: path.path())  // encoded  ❌

path(forKey:) is cacheRoot.appendingPathComponent("\(key).cache.json") with the key unhashed, so a key containing a space or a % would pass the first check and throw on the second.

Fix

1. Compare against the expiry correctly

< instead of >, with a comment naming what the branch means.

2. Read the creation date off the URL

path.resourceValues(forKeys: [.creationDateKey]) instead of attributesOfItem(atPath: path.path()). This matches the fileExists(at:) call above it and removes the encoding question rather than restating it.

3. Take the cache root as an init parameter

public init(cacheRoot: URL = .cachesDirectory). Defaulted, so DiskCache.shared, DiskCache() in AccountHelper, and CachedAndFetchedResult are unchanged. Without it the tests would read, write, and removeAll() in the developer's real caches directory.

Test plan

• Added DiskCacheTests — 8 cases covering store/read round-trip, a fresh entry inside the window, an expired entry, no interval, a missing key, a key needing percent-encoding, remove(key:), and removeAll().
• Verified the three bug-targeting cases fail without the fix and the five controls pass either way.
• swift test --filter DiskCacheTests — 8/8 pass on macOS.

Each test writes to its own UUID-named directory under temporaryDirectory and removes it afterwards.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	34433
	Version	PR #26009
	Bundle ID	org.wordpress.alpha
	Commit	90870bd
	Installation URL	6jjlttnf25bng

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	34433
	Version	PR #26009
	Bundle ID	com.jetpack.alpha
	Commit	90870bd
	Installation URL	61vm5girviet0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.