Dropping unused allocations when query is cancelled

[harshavamsi]

Description

[Describe what this change achieves]

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit d7c32f4.

Path	Line	Severity	Description
sandbox/plugins/analytics-backend-datafusion/src/main/java/org/opensearch/be/datafusion/DataFusionPlugin.java	211	medium	DATAFUSION_MAX_QUERY_EXPORT_FRACTION is declared Dynamic, meaning it can be changed cluster-wide at runtime without a node restart. A privileged attacker (or compromised admin account) could set this fraction near 0 to cause all queries to be rejected immediately, effectively denying all data access across the cluster without touching other settings. The legitimate purpose (scaling with pool size) does not require Dynamic scope.
sandbox/plugins/analytics-backend-datafusion/rust/src/api.rs	956	low	The ResourcesExhausted error message returned to callers reveals exact internal memory configuration values (projected byte count and the resolved cap derived from pool.query.max × fraction). While not directly exploitable, this leaks memory sizing details that could assist an attacker in probing pool boundaries or tuning exfiltration batch sizes to stay below the cap.

The table above displays the top 10 most important findings.

Total: 2 | Critical: 0 | High: 0 | Medium: 1 | Low: 1

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