Core: Fix background thread leak in ScanTaskIterable - #18396

Closes: #16758

Problem

When downstream query engines (such as StarRocks, Trino, or Spark) cancel or abort a REST table scan early due to client disconnects, timeouts, or query limits, they trigger the cleanup sequence on the outer execution container.

In Apache Iceberg, ScanTaskIterable.close() was implemented as an empty no-op method. Because this outer close() call failed to cascade the shutdown signal to the underlying data structures:

• The internal shutdown state atomic flag remained false.
• Background PlanTaskWorker threads continued running indefinitely.
• Once the internal taskQueue reached its 1000 item capacity limit, all active worker threads became permanently deadlocked inside offerWithTimeout(), leading to thread pool exhaustion on the engine coordinator side.

Solution

1. Implemented State Tracking and Cleanup: Added thread-safe execution barriers inside ScanTaskIterable.close() utilizing shutdown.compareAndSet(false, true).
2. Queue Eviction Matrix: Updated the close block to explicitly flush taskQueue, planTasks, and initialFileScanTasks lists upon termination. This allows background threads stuck in an offer wait cycle to instantly unblock, evaluate the flipped shutdown state, and exit gracefully.
3. Decoupled Iterator Lifecycle: Refactored the internal ScanTasksIterator.close() block to eliminate redundant code duplication, rewriting it to delegate its cleanup tasks straight up to ScanTaskIterable.this.close(). This ensures unified thread termination safety across all potential entry points.
4. Regression Test Addition: Designed and integrated TestScanTaskIterableLeak under the org.apache.iceberg.rest test package, proving that active planning thread allocations successfully scale back down to 0 upon premature termination.

Verification Testing

# 1. Clean format code using Spotless rules
./gradlew spotlessApply

# 2. Run static quality analysis lint checks on modified packages
./gradlew :iceberg-core:compileJava :iceberg-core:compileTestJava

# 3. Verify the core build pass and execute the regression test case
./gradlew :iceberg-core:test --tests "org.apache.iceberg.rest.TestScanTaskIterableLeak" --info