[BUG] PPL returns HTTP 200 with silently partial results when shards fail (no _shards failure surfaced)

Describe the bug

When shards fail (e.g. Workload Management rejects queries at the shard level, or circuit-breaker/timeout on some shards), OpenSearch returns HTTP 200 with a _shards block such as {total:1000, successful:12, failed:988}. PPL discards the _shards info and returns the partial result as if it were complete — no error, no warning. Users get silently incorrect answers during load spikes (e.g. a returned count of 50 might really be 5000). DSL surfaces the shard failures; PPL does not.

To Reproduce

Run a PPL aggregation on a large multi-shard index under enough load (or WLM CPU enforcement) that some shards are rejected. Compare the PPL count to the equivalent _search DSL response's _shards.failed. PPL reports only the partial count with no indication.

Expected behavior

When failedShards > 0, PPL should either (a) attach a warning naming the shard-failure counts (total/successful/failed), or (b) optionally fail, so the consumer knows the answer is partial.

Root cause

OpenSearchResponse (opensearch module) ingests the SearchResponse but reads only getHits() and getAggregations() — it discards getTotalShards(), getSuccessfulShards(), getFailedShards(), getShardFailures(). The request also does not set allowPartialSearchResults(false), so it inherits the cluster default (true). Identical on the V2 and Calcite scan paths — both funnel through OpenSearchResponse.

Proposed fix

Capture failedShards/successfulShards/totalShards off the SearchResponse in OpenSearchResponse and, when failedShards > 0, raise a structured warning through the response warning channel (the Warning{type,message,detail} → QueryResponse.warnings → SimpleJsonResponseFormatter path added for mapping-conflict partial results). Optionally add a per-request/cluster setting to hard-fail instead of warn (mirrors DSL allow_partial_search_results).

Trace how OpenSearchResponse ingests SearchResponse for both the V2 and Calcite scan paths, then follow the Warning to QueryResponse.warnings and SimpleJsonResponseFormatter path used for mapping-conflict partial results. Capture the shard counts and surface failed-shard information when failures occur; done means PPL no longer presents partial results without an indication.