Fix ResponseFormat="json" for Ollama

Author: ccreutzi

Diff for: +llms/+internal/callOllamaChatAPI.m

@@ -100,7 +100,7 @@
 options = struct;
 
 if strcmp(nvp.ResponseFormat,"json")
-    parameters.format = struct('type','json_object');
+    parameters.format = "json";
 elseif isstruct(nvp.ResponseFormat)
     parameters.format = llms.internal.jsonSchemaFromPrototype(nvp.ResponseFormat);
 elseif startsWith(string(nvp.ResponseFormat), asManyOfPattern(whitespacePattern)+"{")

Diff for: tests/tollamaChat.m

@@ -61,6 +61,12 @@ function generateOverridesProperties(testCase)
             testCase.verifyThat(text, EndsWithSubstring("3, "));
         end
 
+        function generateJSON(testCase)
+            testCase.verifyClass( ...
+                generate(testCase.defaultModel,"create some address, return json",ResponseFormat="json"), ...
+                "string");
+        end
+
         function generateWithToolsAndStreamFunc(testCase)
             % The test point in htoolCalls expects a format that is
             % different from what we get from Ollama. Having that