Cancel background batch producers before collecting final plan metrics #5504

Describe the bug

The async I/O execution path discards its producer JoinHandle. When Spark closes a plan early, releasePlan snapshots metrics before the receiver and context are dropped. The producer can still own the stream, so stream-drop metric guards can update counters only after the final snapshot. Closing the consumer also does not explicitly request producer cancellation.

Steps to reproduce

Start an async batch producer whose stream owns a drop-time metric guard and remains pending, then close its consumer before it finishes. A final snapshot taken before the producer releases the stream misses the guard's update. A producer inside synchronous decoding is a distinct case: it cannot observe cancellation until it yields.

Expected behavior

Retain producer ownership, close the receiver, request cancellation, and wait only within a bounded budget before the final metric snapshot. Cooperative completion should expose drop-time updates; timeout must explicitly remain a potentially incomplete snapshot. There must be no claim that a timeout makes external JVM task state safe to tear down.

Additional context

Extracted from #5453 to review cancellation behavior and close latency independently of new I/O counters. Existing producer ownership is async move over the stream and sender, not a captured raw ExecutionContext pointer. Cancellation remains cooperative; this issue does not promise preemption of synchronous decoding or complete accounting after a timeout.