add annotation for the 400 response

Author: emmalanigan

src/main/kotlin/uk/gov/justice/digital/hmpps/hmppsintegrationapi/controllers/v1/prison/PrisonController.kt

@@ -41,6 +41,11 @@ class PrisonController(
     description = "<b>Applicable filters</b>: <ul><li>prisons</li></ul>",
     responses = [
       ApiResponse(responseCode = "200", useReturnTypeSchema = true, description = "Successfully found a prisoner with the provided HMPPS ID."),
+      ApiResponse(
+        responseCode = "400",
+        description = "The HMPPS ID provided has an invalid format.",
+        content = [Content(schema = Schema(ref = "#/components/schemas/BadRequest"))],
+      ),
       ApiResponse(responseCode = "404", content = [Content(schema = Schema(ref = "#/components/schemas/PersonNotFound"))]),
       ApiResponse(responseCode = "500", content = [Content(schema = Schema(ref = "#/components/schemas/InternalServerError"))]),
     ],