fix(ext-openapi): an answer's media type is the handler's, not the body's

@Consumes says what a route reads. It was also deciding what the document said
a route answers, through a JSON default nothing could override: a handler that
takes a JSON body is not thereby a handler that answers JSON.

@Produces now says that, beside @Consumes and as descriptive as it is — on the
handler, or once on a base class. Every response takes its media type from it,
JSON when nothing declares one, and the error object stays JSON because that is
what Flash answers a failure with whatever the route produces.

Content loses contentType with it. One handler answers in one format and a
status code does not change that, so the media type was in the wrong place; a
response with no schema and no return type to infer one from is a response
with no body, which is what a 204 was using it to say.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flash-extensions/flash-ext-jackson-json/src/test/java/dev/relism/flash/ext/jackson/json/ConstraintsInTheDocumentTest.java

@@ -33,7 +33,7 @@ class ConstraintsInTheDocumentTest {
 
     @GET("/accounts")
     @ApiOperation(summary = "List accounts")
-    @APIResponse(responseCode = "200", content = @Content(contentType = ContentType.JSON, schema = Account.class))
+    @APIResponse(responseCode = "200", content = @Content(schema = Account.class))
     public static class ListAccounts extends RequestHandler {
         @Override public Object handle(Request request, Response response) {
             return new Account("alice", "a@b.com", 30);