Kafka Streams runner: merging windows and custom WindowFns

Two related windowing gaps in the Kafka Streams runner.

**Merging windows.** Session windows do not work. `ReduceFnRunner` drives merging through a merging window set that moves per-window state as windows merge, which the runner's state backing does not implement. `GroupByKeyTest$WindowTests.testGroupByKeyMergingWindows` and `CombineTest$WindowingTests.testSessionsCombine` are sickbayed for this.

**Custom WindowFns.** The runner rebuilds the standard WindowFns from their URNs and interprets them directly, which is what makes a pipeline from any SDK work. A WindowFn the user wrote is opaque to the runner and would have to be executed through the SDK harness that owns it.

Part of #18479.

Start with ReduceFnRunner, the Kafka Streams runner's state backing, and the merging-window behavior described in GroupByKeyTest$WindowTests.testGroupByKeyMergingWindows and CombineTest$WindowingTests.testSessionsCombine. Reproduce the sickbayed session-window tests first; done requires merging windows to work and a defined path for custom WindowFns through the owning SDK harness.

Written by the indexing model from the issue text.