Search: Fix ingest timestamp access in content-date pipeline

Replace ctx._ingest.timestamp (not available in Painless script
processors) with a set processor using Mustache {{{_ingest.timestamp}}}.
The set processor pre-sets content_last_updated to the ingest timestamp,
and the script processor only overwrites it when the enrich lookup finds
a matching content hash.

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

Author: reakaleek

src/Elastic.Markdown/Exporters/Elasticsearch/ContentDateEnrichment.cs

@@ -141,6 +141,14 @@ private async Task PutPipelineAsync(Cancel ct)
 		{
 			["description"] = "Resolves content_last_updated via enrich policy lookup on content_hash",
 			["processors"] = new JsonArray(
+				new JsonObject
+				{
+					["set"] = new JsonObject
+					{
+						["field"] = "content_last_updated",
+						["value"] = "{{{_ingest.timestamp}}}"
+					}
+				},
 				new JsonObject
 				{
 					["enrich"] = new JsonObject
@@ -161,8 +169,6 @@ private async Task PutPipelineAsync(Cancel ct)
 							def lookup = ctx._content_date_lookup;
 							if (lookup != null && lookup.content_hash != null && lookup.content_hash == ctx.content_hash) {
 								ctx.content_last_updated = lookup.content_last_updated;
-							} else {
-								ctx.content_last_updated = ctx._ingest.timestamp;
 							}
 							ctx.remove('_content_date_lookup');
 							"""