fix(flags): dedupe exposures by latest assignment

[leoromanovsky]

Motivation

Android flags exposure deduplication currently treats every previously seen targetingKey + flagName + allocationKey + variationKey tuple as permanently sent. That suppresses valid exposure events when the same subject and flag cycles assignments, for example A -> B -> A, because the final A was seen earlier.

Related iOS PR: DataDog/dd-sdk-ios#2987 applies the same mobile exposure-cache contract.

Changes

• Cache exposures by targetingKey + flagName.
• Store the latest allocationKey + variationKey as the cache value.
• Emit an exposure when the subject/flag has no cached value or when the latest assignment differs from the cached value.
• Keep a bounded Android LruCache, but make it entry-count based with a 5,000-entry limit instead of using approximate object-size accounting.
• Add a comment documenting the expected high-water mark of two subjects and 2,500 flags each; normal flag keys are typically tens of characters, so count-based sizing is easier to reason about.
• Add a regression test proving A -> B -> A writes three exposure events.
• Update the existing mixed scenario to expect a cycle-back exposure.

Decisions

• Kept the change internal to the processor with no public API changes.
• Preserved an LRU bound for apps with many flags moving through a client process, while sizing it directly around the expected two-subject, 2,500-flag high-water mark.
• Avoided approximate byte accounting in sizeOf; AndroidX LruCache defaults to one unit per entry when sizeOf is not overridden.
• Matched the iOS PR's count-based mobile policy while using Android's platform cache primitive.
• Kept synchronization around cache check/update so duplicate suppression remains atomic.
• Validation: focused regression test failed before the implementation, then passed after the fix. JAVA_HOME=/opt/homebrew/Cellar/openjdk@17/17.0.19/libexec/openjdk.jdk/Contents/Home ANDROID_HOME=/opt/homebrew/share/android-commandlinetools ./gradlew :features:dd-sdk-android-flags:testDebugUnitTest passed.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.19%. Comparing base (8b031cb) to head (416ee2d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3526      +/-   ##
===========================================
- Coverage    72.22%   72.19%   -0.03%     
===========================================
  Files          965      965              
  Lines        35635    35630       -5     
  Branches      5947     5947              
===========================================
- Hits         25736    25723      -13     
- Misses        8282     8289       +7     
- Partials      1617     1618       +1     

Files with missing lines	Coverage Δ
.../android/flags/internal/ExposureEventsProcessor.kt	100.00% <100.00%> (ø)

... and 36 files with indirect coverage changes

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[dd-octo-sts-6cbbf8]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep cache updates ordered with exposure writes

When the same subject/flag is evaluated concurrently with different assignments, this cache update can happen before the previous exposure is actually written. For example, thread A caches A and pauses, thread B caches and writes B, then thread A writes A; the cache still says B, so the next B exposure is suppressed even though the latest emitted assignment was A. The new latest-assignment contract only holds if updating the cache and emitting the exposure remain ordered for a given cache key.

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