Normalize required fields in agent JSON schemas for OpenAI

OpenAI rejects schemas where optional properties are absent from the
required array. Promote all properties to required and mark formerly
optional ones nullable so the model knows it may pass null.

Also upgrade tool error log level from Warn to Error.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/agent/tool_test.go

@@ -128,7 +128,7 @@ func TestFunctionTool_Definition(t *testing.T) {
 	)
 
 	t.Run(
-		"pointer fields are not nullable in schema",
+		"omitempty fields are nullable and in required",
 		func(t *testing.T) {
 			t.Parallel()
 
@@ -147,10 +147,16 @@ func TestFunctionTool_Definition(t *testing.T) {
 			var schema map[string]any
 			require.NoError(t, json.Unmarshal(tool.Definition().Parameters, &schema))
 
+			// OpenAI requires all properties to be in required; optional
+			// fields are represented as nullable.
+			required := schema["required"].([]any)
+			assert.Contains(t, required, "title")
+
 			props := schema["properties"].(map[string]any)
 			titleProp := props["title"].(map[string]any)
-			assert.Equal(t, "string", titleProp["type"])
-			assert.Nil(t, titleProp["types"])
+			titleType := titleProp["type"].([]any)
+			assert.Contains(t, titleType, "string")
+			assert.Contains(t, titleType, "null")
 		},
 	)
 }