fix: preserve search_after position when retrying with new PIT

When a PIT expired mid-pagination, the retry was resetting the cursor
to null, silently restarting from the beginning. Now the retry preserves
the original search_after sort values and only replaces the PIT ID.

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

Author: reakaleek

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

@@ -35,7 +35,8 @@ public async Task<ChangesResult> GetChangesAsync(ChangesRequest request, Cancel
 				{
 					LogPitExpired(logger);
 					pitId = await OpenPit(ctx);
-					response = await Search(request with { Cursor = null }, pitId, fetchSize, ctx);
+					var updatedCursor = request.Cursor with { PitId = pitId };
+					response = await Search(request with { Cursor = updatedCursor }, pitId, fetchSize, ctx);
 				}
 
 				if (!response.IsValidResponse)
@@ -178,6 +179,6 @@ private static ChangesResult BuildResult(SearchResponse<DocumentationDocument> r
 	[LoggerMessage(Level = LogLevel.Debug, Message = "Opened new PIT: {PitId}")]
 	private static partial void LogPitOpened(ILogger logger, string pitId);
 
-	[LoggerMessage(Level = LogLevel.Warning, Message = "PIT expired or not found, opening a new one and retrying without search_after")]
+	[LoggerMessage(Level = LogLevel.Warning, Message = "PIT expired or not found, opening a new one and retrying with existing search_after position")]
 	private static partial void LogPitExpired(ILogger logger);
 }