Search: Fix content date enrichment initialization for existing environments

The staging index + alias swap pattern introduced in #3098 fails in
environments that already have a concrete index with the alias name.
This adds a migration path that reindexes data into a timestamped
backing index, removes the legacy index, and creates the alias.

Also makes enrich policy creation idempotent (delete-before-put) and
adds a last_updated fallback in the sitemap reader for documents
indexed before the content date pipeline existed.

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

Author: reakaleek

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

@@ -106,11 +106,39 @@ private async Task EnsureLookupIndexAsync(Cancel ct)
 
 		var indexName = GenerateStagingName();
 		await CreateLookupIndexAsync(indexName, ct);
-		await SwapAliasAsync(null, indexName, ct);
 
+		// A concrete index may exist with the alias name from before the alias-swap pattern.
+		// Preserve its data in the new timestamped index and remove it so the alias can be created.
+		if (await IndexExistsAsync(_lookupAlias, ct))
+		{
+			logger.LogInformation("Migrating legacy concrete index {Alias} to alias-backed pattern", _lookupAlias);
+			await ReindexToLookupAsync(_lookupAlias, indexName, ct);
+			await RefreshIndexAsync(indexName, ct);
+
+			var deleteResponse = await operations.WithRetryAsync(
+				() => transport.DeleteAsync<StringResponse>(_lookupAlias, new DefaultRequestParameters(), PostData.Empty, ct),
+				$"DELETE {_lookupAlias}",
+				ct
+			);
+			if (!deleteResponse.ApiCallDetails.HasSuccessfulStatusCode)
+				throw new InvalidOperationException(
+					$"Failed to delete legacy index {_lookupAlias} during migration: {deleteResponse.ApiCallDetails.DebugInformation}");
+		}
+
+		await SwapAliasAsync(null, indexName, ct);
 		logger.LogInformation("Created content date lookup index {Index} with alias {Alias}", indexName, _lookupAlias);
 	}
 
+	private async Task<bool> IndexExistsAsync(string indexName, Cancel ct)
+	{
+		var response = await operations.WithRetryAsync(
+			() => transport.GetAsync<StringResponse>(indexName, ct),
+			$"GET {indexName}",
+			ct
+		);
+		return response.ApiCallDetails.HasSuccessfulStatusCode;
+	}
+
 	private async Task CreateLookupIndexAsync(string indexName, Cancel ct)
 	{
 		var mapping = new JsonObject
@@ -180,6 +208,13 @@ private async Task DeleteIndexAsync(string indexName, Cancel ct)
 
 	private async Task PutEnrichPolicyAsync(Cancel ct)
 	{
+		// Enrich policy PUT does not support upsert; delete first if it already exists.
+		_ = await operations.WithRetryAsync(
+			() => transport.DeleteAsync<StringResponse>($"/_enrich/policy/{PolicyName}", new DefaultRequestParameters(), PostData.Empty, ct),
+			$"DELETE _enrich/policy/{PolicyName}",
+			ct
+		);
+
 		var policy = new JsonObject
 		{
 			["match"] = new JsonObject

src/services/Elastic.Documentation.Assembler/Building/EsSitemapReader.cs

@@ -61,7 +61,8 @@ public async IAsyncEnumerable<SitemapEntry> ReadAllAsync([EnumeratorCancellation
 							continue;
 
 						var url = source["url"]?.GetValue<string>();
-						var lastUpdatedStr = source["content_last_updated"]?.GetValue<string>();
+						var lastUpdatedStr = source["content_last_updated"]?.GetValue<string>()
+							?? source["last_updated"]?.GetValue<string>();
 
 						if (url is null || lastUpdatedStr is null)
 							continue;
@@ -131,7 +132,7 @@ internal static string BuildSearchBody(string pitId, string[]? searchAfter)
 		var body = new JsonObject
 		{
 			["size"] = PageSize,
-			["_source"] = new JsonArray("url", "content_last_updated"),
+			["_source"] = new JsonArray("url", "content_last_updated", "last_updated"),
 			["query"] = new JsonObject
 			{
 				["bool"] = new JsonObject