fix: cancel in-flight initialize when context changes

[fabriziodemaria]

Summary

• Route initialize and onContextSet through the same TaskManager generation so a later context change cancels in-flight init/prefetch instead of racing it.
• Emit .ready / .contextChanged / .stale only if that call is still current; a superseded fetch cannot overwrite flags or status.
• Add overlap tests for fetchAndActivate init, activateAndFetchAsync prefetch, and stacked onContextSet calls.

Why not FIFO

OpenFeature 0.6 serializes lifecycle call starts, not Future completion. #261 queues the work so initialize always finishes first. This PR follows the SDK's existing cancel-and-supersede model (and OpenFeature's guidance) so the app does not wait on a resolve for a context it already replaced.

Stacked on #259.

Test plan

• swift test --skip ConfidenceIntegrationTest --skip MixedTypesFlagIntegrationTest
• swiftlint lint --strict on touched files
• CI (Unit-Tests, Integration-Tests, API-diff, SwiftLint)

Made with Cursor

[nicklasl]

closes #262

[nicklasl]

Could we make committing the resolution conditional on the task still owning the current generation? I guess cancellation can land after the check but before storage.save, which lets a superseded request overwrite newer persisted flags; I pushed eda89fd with a deterministic failing test that reloads 3 instead of 7.