fix(connectivity_plus): prevent Linux client leaks

[Kibiandkimi]

Description

Fix the Linux NetworkManagerClient lifecycle when connectivity listeners are canceled and immediately re-subscribed.

StreamController.broadcast does not wait for an asynchronous onCancel callback. The previous implementation accessed the shared _client on both sides of await calls, so a new listener could reuse a client that was still closing, while an older listener setup could later dereference a cleared client. Closing a NetworkManagerClient before its pending connection finishes can also leave the subsequently opened system D-Bus connection unclosed. In addition, the propertiesChanged subscription was not retained or canceled.

This change:

• owns each client, connection future, and property subscription in one session;
• detaches the current session synchronously before asynchronous cleanup;
• waits for an in-flight connection before closing its client;
• ignores callbacks from stale sessions and cancels property subscriptions;
• closes one-shot clients in checkConnectivity on error paths as well.

The regression tests deterministically cover error cleanup, property-subscription cleanup, and cancellation/re-subscription while a connection is pending.

Related Issues

None.

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

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

• 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.

Validation

• Scoped formatting check for all connectivity_plus packages
• Scoped analyzer for connectivity_plus, connectivity_plus_platform_interface, and the example
• Unit tests for connectivity_plus and connectivity_plus_platform_interface
• Linux example debug build
• Linux NetworkManager integration smoke test