Fix PPL REST handler returning 500 for OpenSearchRejectedExecutionException

[aravindsagar]

Description

Description

RestPPLQueryAction.getRawErrorCode() has a hardcoded return 500 fallback for exceptions that don't match OpenSearchException or the known client-error list. OpenSearchRejectedExecutionException extends RejectedExecutionException → RuntimeException (not OpenSearchException), so it always fell through to the 500 path — even though OpenSearch core's ExceptionsHelper.status() already maps it to TOO_MANY_REQUESTS (429).

This affects any backend that throws OpenSearchRejectedExecutionException through the PPL query path (e.g., admission control rejections from the analytics engine under memory pressure).

Changes

• Replace return 500 with return ExceptionsHelper.status(ex).getStatus() — delegates to the same mapping used by DSL _search and the rest of OpenSearch
• Remove a stale comment about future ErrorCode-based mapping
• Add RestPPLQueryActionTest with unit tests covering the key exception→status mappings

Testing

Unit tests (all pass):

• testRejectedExecutionReturns429 — the primary fix scenario
• testOpenSearchStatusExceptionPreservesStatus — verifies OpenSearchException path still works (404)
• testIndexNotFoundReturns404 — IndexNotFoundException (extends OpenSearchException)
• testSyntaxCheckExceptionReturns400 — client error path
• testQueryEngineExceptionReturns400 — client error path
• testGenericRuntimeExceptionReturns500 — unknown exceptions still return 500

Full plugin test suite: ./gradlew :opensearch-sql-plugin:test — all existing tests pass.

Manual end-to-end verification (local single-node cluster with DataFusion analytics backend):

1. Injected a Rust-side memory admission rejection (admission_rejected_error) in the native query engine
2. NativeErrorConverter correctly converted the native error string to OpenSearchRejectedExecutionException
3. Before fix: PPL /_plugins/_ppl returned HTTP 500 / INTERNAL_SERVER_ERROR
4. After fix: PPL /_plugins/_ppl returned HTTP 429 / TOO_MANY_REQUESTS
5. DSL /_search path confirmed returning 429 both before and after (it already uses ExceptionsHelper.status())

Related Issues

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

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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