[Bug][Spark] SparkStructuredStreamingRunner cancel() does not cancel Spark jobs and stops a SparkSession it may not own

`SparkStructuredStreamingPipelineResult.cancel()` interrupts the execution thread, which does not cancel a Spark job (`DAGScheduler.runJob` only ends through `JobWaiter.cancel`), then stops the SparkSession even when `SparkSession.Builder.getOrCreate` returned a pre-existing default session shared with other pipelines in the JVM. Batch `EvaluationContext.stop()` is a no-op.

Consequences:
- With `useActiveSparkSession=true` a batch pipeline is never cancelled and `cancel()` still returns CANCELLED.
- Two pipelines sharing the default session: cancelling one stops the SparkContext under the other.
- The execution thread keeps translating on a stopped SparkContext. In `beam_PreCommit_Java_Spark_Versions` on #40090 this surfaced as a `NoClassDefFoundError: Could not initialize class PipelineTranslatorBatch` cascade over 12 batch tests in one fork, the primary trace was not captured.

Fix: job group cancellation in `EvaluationContext` (`setJobGroup` on the execution thread, `cancelJobGroup` in `stop()`), session ownership with a reference count in `SparkSessionFactory`, session stop on the execution thread after evaluation, unbounded join in `cancel()` instead of `Future.cancel(true)`.

Known blocker for declaring the Spark 4 Structured Streaming runner stable, #36841. Found in review of #40090.

Start by tracing SparkStructuredStreamingPipelineResult.cancel() through EvaluationContext and SparkSessionFactory, including the execution-thread and shared-session paths described in the issue. Run the relevant Spark runner tests, then the beam_PreCommit_Java_Spark_Versions suite; done means job cancellation works, shared sessions remain usable, and cancellation waits for evaluation to finish.

Written by the indexing model from the issue text.