feat(analytics,ios): add UIScene support for campaign attribution

[SelaseKay]

Description

Under the UIScene lifecycle, UIKit delivers URL and universal-link opens to UISceneDelegate instead of UIApplicationDelegate. Firebase Analytics only swizzles AppDelegate, so firebase_campaign was silently dropped after a host app adopted UIScene. Other Analytics events kept working; the loss is only visible in BigQuery.

This registers the iOS plugin as a scene delegate (when the Flutter SDK provides addSceneDelegate) and forwards:

• scene:willConnectToSession:options: (cold start) to Analytics.handleOpen and Analytics.handleUserActivity
• scene:openURLContexts: to Analytics.handleOpen
• scene:continueUserActivity: to Analytics.handleUserActivity

The scene methods return false so Auth, Messaging, and other plugins still receive the same events. The plugin does not conform to FlutterSceneLifeCycleDelegate at compile time so it still builds against Flutter 3.27; the methods use matching @objc selectors instead.

Automated Dart tests cannot observe firebase_campaign (native UIKit callbacks, BigQuery-only event). Verification remains the issue repro: UIScene host app, simctl openurl with UTM params, BigQuery export.

Related Issues

• Fixes [firebase_analytics]: campaign attribution (firebase_campaign) silently stops working on iOS under the UIScene lifecycle #18572

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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