fix: sample schema should stringify string values for content-type: text/json (#6431)

* reverts #6412 which stringified string value if content-type: text/plain 
* enable stringify string values matching `/json/` and allowList of `shouldStringifyTypes` ("string")

test/unit/core/utils.js

@@ -1494,7 +1494,18 @@ describe("utils", () => {
       Date = oriDate
     })
     
-    it("should stringify string values", () => {
+    it("should stringify string values if json content-type", () => {
+      // Given
+      const res = getSampleSchema({
+        type: "string",
+        format: "date-time"
+      }, "text/json")
+      
+      // Then
+      expect(res).toEqual(JSON.stringify(new Date().toISOString()))
+    })
+
+    it("should not unnecessarily stringify string values for other content-types", () => {
       // Given
       const res = getSampleSchema({
         type: "string",
@@ -1502,7 +1513,7 @@ describe("utils", () => {
       })
       
       // Then
-      expect(res).toEqual(JSON.stringify(new Date().toISOString()))
+      expect(res).toEqual(new Date().toISOString())
     })
 
     it("should not unnecessarily stringify non-object values", () => {
@@ -1514,6 +1525,16 @@ describe("utils", () => {
       // Then
       expect(res).toEqual(0)
     })
+
+    it("should not unnecessarily stringify non-object values if content-type is json", () => {
+      // Given
+      const res = getSampleSchema({
+        type: "number"
+      }, "application/json")
+      
+      // Then
+      expect(res).toEqual(0)
+    })
   })
   
   describe("paramToIdentifier", () => {