refactor: Remove AppDelegateProxy and Related Code

[BrandonStalnaker]

Background

• The AppDelegateProxy feature allowed the mParticle SDK to automatically intercept UIApplicationDelegate messages using NSProxy to handle application notifications (push notifications, URL handling, user activities, etc.) without requiring developers to manually call SDK methods from their app delegate.
• This feature added complexity and was often disabled by customers who preferred explicit control over their app delegate lifecycle.
• Removing this feature simplifies the SDK, reduces potential edge cases, and aligns with the expectation that developers will explicitly forward app delegate events to the SDK.

What Has Changed

• Removed MPAppDelegateProxy class (proxy that intercepted app delegate messages)
• Removed MPSurrogateAppDelegate class (surrogate that implemented delegate methods)
• Removed proxyAppDelegate property from MParticleOptions
• Removed proxiedAppDelegate property from MParticle
• Removed proxyOriginalAppDelegate and unproxyOriginalAppDelegate methods from MPBackendController
• Updated MPBackendController.startWithKey: to no longer accept a proxyAppDelegate parameter
• Updated documentation for app notification methods (didReceiveRemoteNotification:, didRegisterForRemoteNotificationsWithDeviceToken:, etc.) to reflect that they should always be called manually
• Removed the automatic early-return behavior in notification methods that previously skipped processing when the proxy was active
• Updated unit tests to remove proxy-related test cases and assertions
• The SDK now behaves as if proxyAppDelegate was always set to NO - developers must explicitly call the SDK's notification methods from their app delegate

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://rokt.atlassian.net/browse/SDKE-767

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	1.67 MB	1.64 MB	-24 KB
Executable Impact	896 bytes	896 bytes	+N/A
XCFramework Size	9.12 MB	9.06 MB	-60 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/9.0-Release):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1792,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1708,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9336}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1768,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1684,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9276}

[nickolas-dimitrakas]

LGTM, TBD on whether to remove trunk rules for MIGRATING.md