fix(flags): dedupe exposures by latest assignment

[leoromanovsky]

Motivation

DatadogFlags exposure deduplication stored the full exposure tuple as already seen. That suppressed a valid exposure when a subject and flag cycled back to a previous assignment, for example A -> B -> A, because the final A had been seen before.

The cache also needs to protect mobile devices while supporting the expected client high-water mark of two subjects across 2,500 flags.

Related Android PR: DataDog/dd-sdk-android#3526 applies the same mobile exposure-cache contract.

Changes

• Add a regression test proving A -> B -> A writes three exposure events.
• Add focused exposure tracker tests covering repeated assignments, assignment cycles, and 2 subjects x 2,500 flags.
• Change the exposure tracker cache to use targetingKey + flagKey as the key and allocationKey + variationKey as the latest cached value.
• Keep duplicate suppression only for repeated evaluations of the current assignment.
• Use platform NSCache with a 5,000-entry countLimit for the in-memory exposure assignment cache.

Decisions

• Kept the change internal with no public API changes.
• Used a count limit, not a byte limit, to avoid approximate object-size accounting while preserving a reviewer-friendly bound.
• Sized the count limit around the expected two-subject, 2,500-flag high-water mark.
• Used iOS's platform cache primitive instead of a custom deterministic LRU implementation; eviction order is intentionally not part of the correctness contract.
• Matched the Android PR's count-based mobile policy while using iOS's platform cache primitive.
• Matched dd-trace-go and openfeature-js-client semantics, where the dedupe key and assignment value are modeled separately so assignment changes re-emit exposures.

Validation:

• xcodebuild -workspace Datadog.xcworkspace -scheme DatadogFlags -destination "platform=iOS Simulator,id=32AE07AE-F88E-4FF4-A384-F6A347AA3B98" -only-testing:DatadogFlagsTests/ExposureTrackerTests -only-testing:DatadogFlagsTests/ExposureLoggerTests/testLogExposureAssignmentCycle test
• SKIP_LINT=1 xcodebuild -workspace Datadog.xcworkspace -scheme DatadogFlags -destination "platform=iOS Simulator,id=32AE07AE-F88E-4FF4-A384-F6A347AA3B98" test

[dd-octo-sts-03ec73]

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[barboraplasovska]

Looks great! But you should add a CHANGELOG entry.

[barboraplasovska]

LGTM