fix: throw on failed Elasticsearch response instead of returning empty data

Previously, a failed search response was logged as a warning but
execution continued to BuildResult, returning HTTP 200 with
empty/truncated data. Now throws InvalidOperationException so the
failure surfaces as an error to the caller.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: reakaleek

src/services/Elastic.Documentation.Search/ChangesGateway.cs

@@ -41,8 +41,10 @@ public async Task<ChangesResult> GetChangesAsync(ChangesRequest request, Cancel
 
 				if (!response.IsValidResponse)
 				{
-					logger.LogWarning("Elasticsearch changes query returned invalid response. Reason: {Reason}",
-						response.ElasticsearchServerError?.Error.Reason ?? "Unknown");
+					var reason = response.ElasticsearchServerError?.Error.Reason ?? "Unknown";
+					throw new InvalidOperationException(
+						$"Elasticsearch changes query failed (HTTP {response.ApiCallDetails?.HttpStatusCode}): {reason}"
+					);
 				}
 			}