fix: Register background search tasks as children of parent

[Swiddis]

Description

OpenSearch automatically propagates task cancellation to any child tasks, but we weren't actually marking our search tasks as children of the background search process. This led to zombie tasks being left behind on query cancellation.

Related Issues

Resolves #5647

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[github-actions]

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🧪 PR contains tests

🔒 No security concerns identified

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⚡ No major issues detected

[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Remove redundant boolean flag The restored flag is redundant since it's always set to true when cancellableTask != null. Simplify the logic by checking cancellableTask != null directly in the finally block to reduce unnecessary state tracking. opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/BackgroundSearchScanner.java [134-146] private OpenSearchResponse backgroundSearch(ProfileContext ctx, OpenSearchRequest request) { - boolean restored = false; if (cancellableTask != null) { OpenSearchQueryManager.setCancellableTask(cancellableTask); - restored = true; } try { return QueryProfiling.withCurrentContext(ctx, () -> client.search(request)); } finally { - if (restored) { + if (cancellableTask != null) { OpenSearchQueryManager.clearCancellableTask(); } } } Suggestion importance[1-10]: 4 __ Why: The suggestion correctly identifies that the restored flag is redundant since it mirrors the cancellableTask != null check. However, this is a minor code style improvement that doesn't significantly impact functionality, correctness, or maintainability. The existing code is clear and the optimization is marginal.	Low

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.86%. Comparing base (8e83f05) to head (c2db3b2).
⚠️ Report is 10 commits behind head on main.

❌ Your project check has failed because the head coverage (63.86%) is below the target coverage (99.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5888      +/-   ##
============================================
+ Coverage     63.65%   63.86%   +0.21%     
- Complexity     9021     9097      +76     
============================================
  Files           959      962       +3     
  Lines         40823    41057     +234     
  Branches       4634     4671      +37     
============================================
+ Hits          25986    26223     +237     
+ Misses        13964    13956       -8     
- Partials        873      878       +5     

Flag	Coverage Δ
sql-engine	63.86% <100.00%> (+0.21%)	⬆️

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