Use Windows list manager events

[edde746]

Summary

• Subscribe to IID_INetworkListManagerEvents on Windows and implement ConnectivityChanged.
• Clean up partial listener state after failed setup and preserve the HRESULT for useful errors.

Why

INetworkListManagerEvents is the documented sink for machine-level connectivity changes. The previous IID_INetworkEvents subscription can fail during EventChannel activation and surface as PlatformException(NetworkManager::StartListen).

Fixes #3713.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• 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 (flutter analyze) does not report any problems on my PR.

Breaking Change

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• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

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