chore: [SDK-4213] remove background-threading FF, make async init default

[abdulraqeeb33]

Summary

The sdk_background_threading remote feature flag is fully rolled out (100%), so this removes the flag entirely and makes the background/async init path the unconditional default. This eliminates the FF/cached-config read from the init decision path (the StrictMode bootstrap disk-read goal) and removes the legacy synchronous-blocking code path.

Resolves SDK-4213.

Production changes

• FeatureFlag.kt — removed the SDK_BACKGROUND_THREADING enum entry. The generic FeatureActivationMode framework and SDK_IDENTITY_VERIFICATION remain.
• ThreadingMode.kt — deleted (it only existed to hold useBackgroundThreading).
• ThreadUtils.kt — collapsed every if (ThreadingMode.useBackgroundThreading) … else … branch to the OneSignalDispatchers path, dropping the legacy thread { runBlocking { … } } / GlobalScope.launch fallbacks. Renamed runOnSerialIOIfBackgroundThreading → runOnSerialIO and updated all call sites.
• OneSignalImp.kt — removed isBackgroundThreadingEnabled, the featureManager field, the injectable ioDispatcher constructor param, and the now-dead requireInitForOperation / warnIfBlockingOnMainThread. initWithContext always dispatches internalInit via suspendifyOnIO (never blocks the caller); login/logout/updateUserJwt/JWT-listener methods always use waitForInit; suspend accessors use OneSignalDispatchers.IO directly.
• StartupService.kt — removed the IFeatureManager lookup; scheduleStart() always launches on OneSignalDispatchers.launchOnDefault.
• OneSignalCrashUploaderWrapper.kt — removed the FF check in start(); always launches on IO (featureManager still injected for OTel telemetry).
• FeatureManager.kt — removed the SDK_BACKGROUND_THREADING side-effect.

Test changes

• Removed FF on/off matrix tests and ThreadingMode toggling across the affected suites.
• Renamed ThreadUtilsFeatureFlagTests → ThreadUtilsDispatchTests (tests the now-unconditional dispatch contracts).
• Re-pointed the saturated-IO waitForInit SUCCESS fast-path regression tests (Flutter Remove OSNotificationPayload #1163) to the default async path; removed the obsolete warnIfBlockingOnMainThread test.

Net: +274 / −1004 across 26 files.

Test plan

• compileDebugUnitTestKotlin green across all modules
• Affected unit tests pass (SDKInitTests, OneSignalImpTests, FeatureManagerTests, FeatureFlagTests, ThreadUtilsDispatchTests, BackgroundManagerTests, StartupServiceTests, OneSignalCrashUploaderWrapperTest, SessionServiceTests, FeatureFlagsRefreshServiceTests, NotificationsManagerTests, NotificationPermissionControllerTests)
• detekt passes for :core and :notifications
• Full CI suite

Notes / follow-ups

• The generic FeatureActivationMode.APP_STARTUP mode + latching logic in FeatureManager is retained but is now unused by any flag; could be removed in a follow-up if no APP_STARTUP flags are planned.
• A few telemetry/config/parser tests still use the literal string "sdk_background_threading" as an arbitrary remote-flag fixture (no longer maps to any enum) — left as-is since they exercise generic plumbing.

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📊 Diff Coverage Report

Diff Coverage Report (Changed Lines Only)

Gate: aggregate coverage on changed executable lines must be ≥ 80% (JaCoCo line data for lines touched in the diff).

Changed Files Coverage

• OneSignalDispatchers.kt: 92/109 touched executable lines (84.4%) (174 touched lines in diff)
• ThreadUtils.kt: 15/20 touched executable lines (75.0%) (26 touched lines in diff)
  • 5 uncovered touched lines in this file
• ⚠️ ThreadingMode.kt: Not in coverage report (may not be compiled/tested)
• BackgroundManager.kt: 2/2 touched executable lines (100.0%) (4 touched lines in diff)
• FeatureFlagsRefreshService.kt: 2/2 touched executable lines (100.0%) (3 touched lines in diff)
• FeatureFlag.kt: 9/9 touched executable lines (100.0%) (all lines — could not resolve diff)
• StartupService.kt: 4/6 touched executable lines (66.7%) (6 touched lines in diff)
  • 2 uncovered touched lines in this file
• OneSignalCrashUploaderWrapper.kt: 0/3 touched executable lines (0.0%) (3 touched lines in diff)
  • 3 uncovered touched lines in this file
• OneSignalImp.kt: 1/27 touched executable lines (3.7%) (58 touched lines in diff)
  • 26 uncovered touched lines in this file
• SessionService.kt: 2/2 touched executable lines (100.0%) (5 touched lines in diff)
• NotificationsManager.kt: 0/1 touched executable lines (0.0%) (2 touched lines in diff)
  • 1 uncovered touched lines in this file
• NotificationPermissionController.kt: 0/2 touched executable lines (0.0%) (3 touched lines in diff)
  • 2 uncovered touched lines in this file
• ⚠️ IOMockHelper.kt: Not in coverage report (may not be compiled/tested)

Overall (aggregate gate)

127/183 touched executable lines covered (69.4% — requires ≥ 80%)

Per-file detail (informational; gate is aggregate above):

• ThreadUtils.kt: 75.0% (5 uncovered touched lines)

• StartupService.kt: 66.7% (2 uncovered touched lines)

• OneSignalCrashUploaderWrapper.kt: 0.0% (3 uncovered touched lines)

• OneSignalImp.kt: 3.7% (26 uncovered touched lines)

• NotificationsManager.kt: 0.0% (1 uncovered touched lines)

• NotificationPermissionController.kt: 0.0% (2 uncovered touched lines)

❌ Coverage Check Failed

Aggregate coverage on touched lines is 69.4% (minimum 80%).

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