fix(connectivity_plus): release iOS multi-engine callbacks

[FeliksLv01]

Description

Fixes iOS lifecycle retention in applications that register connectivity_plus on multiple FlutterEngine instances. The plugin and its connectivity provider currently retain each other through bound callback references. PathMonitorConnectivityProvider and NWPathMonitor have the same retention pattern.

This change uses weak callback captures and publishes the plugin instance through the registrar. Flutter can then invoke detachFromEngine when an engine is deallocated, which clears the EventChannel sink and stops the monitor.

Related Issues

• Fixes [Bug]: iOS connectivity_plus retains callbacks in multi-FlutterEngine apps #3958

Checklist

• I read the Contributor Guide and followed the required package-specific branch and PR workflow.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer does not report any problems on this PR.

Validation

• GitHub Actions: Dart Analyzer and iOS example build passed.
• Local: melos run format passed without tracked-file changes.
• Local: flutter pub publish --dry-run for connectivity_plus passed from a clean working tree.
• The CI pub.dev job fails before testing this change: with Flutter 3.47, its workspace bootstrap marks example/analysis_options.yaml modified in android_intent_plus, connectivity_plus, and package_info_plus; pub rejects each dirty package. This PR does not modify those files.
• Platform integration jobs remain the source of truth and are running on the PR.

Breaking Change

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

[FeliksLv01]

Closing this premature submission. I will reopen a validated PR with the required template, test results, and issue context.

[FeliksLv01]

CI note: the remaining failures are unrelated to the iOS callback lifecycle change in this PR. Flutter 3.47 introduced repository-wide toolchain validation and formatting migrations: some example projects require Gradle/Kotlin upgrades, and Dart 3.13 reformats an existing share_plus test. This PR also temporarily touches three package examples to unblock pub validation, which triggers their package-specific workflows; the package_info_plus iOS integration failure is a pre-existing CocoaPods sandbox/Podfile.lock synchronization issue. The connectivity_plus analyzer, unit tests, pub dry-run, and iOS/macOS example builds pass.