add changelog

Author: szybia

docs/changelog/130325.yaml

@@ -0,0 +1,16 @@
+pr: 130325
+summary: "Simulate API: Return 400 on invalid processor(s)"
+area: Ingest Node
+type: breaking
+issues: []
+
+breaking:
+  title: Return 400 on invalid processor(s) in Simulate API
+  area: Ingest Node
+  details: >-
+    In earlier versions of {es}, the Simulate API would return a 500 error when encountering invalid processors. Now, it returns a 400 Bad Request error instead.
+
+  impact: >-
+    Callers should expect a 400 Bad Request response when the Simulate API encounters invalid processors. This change improves error handling and provides clearer feedback on request issues.
+
+  notable: false