chore: [SDK-5307] remove stale compatibility code for iOS below 15

[nan-li]

Description

One Line Summary

Remove the runtime guards and reflection that only existed for iOS 12 through 14, and for iOS 15 on Mac Catalyst. SDK-5307.

Details

Motivation

#1749 raised the deployment target to iOS 15 and #1757 raises Mac Catalyst to match, so every @available(iOS 13.0, *), @available(iOS 14.0, *) and @available(iOS 15.0, *) check is always true, and the performSelector and NSInvocation calls that avoided linking newer APIs at compile time have no reason left to exist.

Scope

Stacked on #1757. The first commit needs only the iOS floor; the second needs the Catalyst floor too.

• The two Objective-C lifecycle observers and OSRemoteLoggingController register UIScene observers with the UIScene* notification constants instead of string names.
• OSBundleUtils.isAppUsingUIScene and the ephemeral authorization check lose their guards and else branches.
• UIApplication+OneSignal and OSMessagingController read the key window's scene through the typed UIWindowScene API. A nil key window still reads as active, as it did through performSelector.
• The attachment downloader calls NSFileHandle writeData:error: directly and drops the pre-13 writeData: fallback.
• createActionForButton: builds UNNotificationActionIcon and calls actionWithIdentifier:title:options:icon: directly, and drops the pre-15 fallback that built the action without an icon. A button without an icon key still passes a nil icon.
• OneSignalCoreMocks and the dev app lose the matching #available checks, and the dev app's willPresentNotification always uses the banner and list options.
• Not in this PR: the keyWindow reads, the Xcode-era AUTH_STATUS_EPHEMERAL and provisional defines, and the commented-out blocks in the legacy UnitTests target.

Testing

Unit testing

No new tests. The removed branches were unreachable at the iOS 15 floor, and the existing lifecycle, in-app messaging and notification settings tests cover the paths that remain.

Manual testing

Reduced test plan run serially on an iPhone 17 Pro Max simulator with Xcode 27.0: 353 tests, 0 failures. build-for-testing produces the same warning set as main. With #1757 underneath, OneSignalFramework builds for Mac Catalyst on Xcode 27.0 at the ios15.0-macabi target. The dev app builds against the resulting frameworks. Not run on a device.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

🤖 Generated with Claude Code

[fadi-george]

Potential issues:

• writeData:error: returns a BOOL, might be cleaner to check that instead of fileHandleError
• OSRemoteLoggingControllerTests still posts "UISceneDidEnterBackgroundNotification" by string, could use UIScene.didEnterBackgroundNotification to match
• NSClassFromString for UNTextInputNotificationResponse / UNPushNotificationTrigger in UNUserNotificationCenter+OneSignalNotifications.m could go too if you want the cleanup to be complete