fix: use .keyword suffix for MDC exchangeId term query

Dynamically mapped string fields in OpenSearch are multi-field
(text + keyword). Term queries require the .keyword sub-field for
exact matching.

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

cameleer3-server-app/src/main/java/com/cameleer3/server/app/search/OpenSearchLogIndex.java

@@ -113,7 +113,7 @@ public class OpenSearchLogIndex {
                 // Match on top-level field (new records) or MDC nested field (old records)
                 bool.must(Query.of(q -> q.bool(b -> b
                         .should(Query.of(s -> s.term(t -> t.field("exchangeId").value(FieldValue.of(exchangeId)))))
-                        .should(Query.of(s -> s.term(t -> t.field("mdc.camel.exchangeId").value(FieldValue.of(exchangeId)))))
+                        .should(Query.of(s -> s.term(t -> t.field("mdc.camel.exchangeId.keyword").value(FieldValue.of(exchangeId)))))
                         .minimumShouldMatch("1"))));
             }
             if (level != null && !level.isEmpty()) {