fix(firestore, android): defer transaction cleanup until stream cancellation

[SelaseKay]

Description

On Android, transaction event channels were unregistered immediately after native completion, so Dart's subsequent stream cancellation reached no handler and reported a MissingPluginException even though the transaction succeeded. This PR defers transaction cleanup until the Dart EventChannel subscription is cancelled, avoids recursive cancellation during plugin-wide listener disposal, and adds Android integration coverage for the teardown path.

The targeted regression test and the complete transaction integration group pass on Android.

Related Issues

Fixes #18546

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Breaking Change

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