fix(battery_plus): invoke client.close() on Linux batteryState getter

[chiliec]

Description

On Linux, the batteryState getter passes a tear-off (client.close) to
whenComplete instead of invoking it (client.close()), so the UPower client
is never closed after reading the battery state. The sibling batteryLevel
getter already calls client.close() correctly — this brings batteryState
in line with it.

// before
.whenComplete(() => client.close);
// after
.whenComplete(() => client.close());

Related Issues

• Fix [Bug]: Linux bug on batteryState getter #3964

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

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

Tests

Added a regression test (battery state closes the client) to
battery_plus_linux_test.dart that verifies client.close() is invoked once
after batteryState completes.

Verified genuine RED→GREEN with Flutter 3.47.2 (Dart 3.13.2) on Linux:

• Before the fix: verify(client.close()).called(1) fails —
  No matching calls. All calls: MockUPowerClient.connect(), MockUPowerClient.displayDevice.
• After the fix: all 5 tests pass.

$ flutter test test/battery_plus_linux_test.dart
00:00 +5: All tests passed!

$ flutter analyze lib/src/battery_plus_linux.dart test/battery_plus_linux_test.dart
No issues found!

Happy to adjust if you'd prefer a different test shape.

---

AI disclosure: this change was prepared with AI assistance and verified locally as described above.

[chiliec]

Heads up on CI: the red Check code formatting job is not from this PR — it only flags packages/share_plus/share_plus/test/share_plus_windows_test.dart, which is unchanged here and already reformats on a clean main checkout (newer dart formatter than the repo was last formatted with). The formatting job reports battery_plus: SUCCESS, and dart format --set-exit-if-changed on the two files this PR touches reports 0 changed.

The test, analyze, Dart Analyzer, linux_example_build and linux_integration_test checks (the ones relevant to this Linux-only Dart change) are green. The remaining red checks (ios/macos/windows example builds, pub.dev requirements) are platform/infra jobs unrelated to this change. Happy to rebase if you'd like.