fix: string validation in variable input schema (#603)

# Background
When trying to input a `jsonPath` value type in standard transformer
page (for variable input & feast table input), the value that is passed
to the yup validation is type of object (e.g. `{ "jsonPath": "$.id" }`),
instead of just the string value of jsonPath (`$.id`) inputted in the
form. This prevent user to submit the form when their actual input is
valid.

<img width="660" alt="Screenshot 2024-08-29 at 13 18 40"
src="https://github.com/user-attachments/assets/8ab9bbd3-c92f-46fa-b3aa-49b4c5ce938c">
<img width="530" alt="Screenshot 2024-08-29 at 13 40 48"
src="https://github.com/user-attachments/assets/d09d13c8-15ca-405e-bcb8-3cc48333ad91">


# Modifications
Use a custom yup test function to validate `value`; if the value passed
is type of object & has the field of `jsonPath`, it will compare the
`jsonPath` value to the original schema; else can just compare the
original value to the yup schema.

# Tests

# Checklist
- [x] Added PR label
- [ ] Added unit test, integration, and/or e2e tests
- [x] Tested locally
- [ ] Updated documentation
- [ ] Update Swagger spec if the PR introduce API changes
- [ ] Regenerated Golang and Python client if the PR introduces API
changes

# Release Notes
<!--
Does this PR introduce a user-facing change?
If no, just write "NONE" in the release-note block below.
If yes, a release note is required. Enter your extended release note in
the block below.
If the PR requires additional action from users switching to the new
release, include the string "action required".

For more information about release notes, see kubernetes' guide here:
http://git.k8s.io/community/contributors/guide/release-notes.md
-->

```release-note
fix variable input validation in transformer config ui page
```

ui/src/pages/version/components/forms/validation/schema.js

@@ -67,6 +67,21 @@ const environmentVariableSchema = yup.object().shape({
   value: yup.string(),
 });
 
+const stringValueSchema = (name, key) => yup.mixed().test(
+  `value input schema`,
+  `${name} must be a \`string\` but the final value was $\{value}`,
+  (value, { createError }) => {
+    try {
+      if (typeof value == "object" && key in value) {
+        return yup.string().required(`${name} is required`).validateSync(value[key])
+      }
+      return yup.string().required(`${name} is required`).validateSync(value)
+    } catch (e) {
+      return createError({ message: e.message })
+    }
+  }
+)
+
 export const versionEndpointSchema = (maxAllowedReplica) => yup.object().shape({
   environment_name: yup.string().required("Environment is required"),
   resource_request: resourceRequestSchema(maxAllowedReplica),
@@ -92,7 +107,7 @@ const feastEntitiesSchema = yup.object().shape({
   name: yup.string().required("Entity Name is required"),
   valueType: yup.string().required("Entity Value Type is required"),
   fieldType: yup.string().required("Input Type is required"),
-  field: yup.string().required("Input Value is required"),
+  field: stringValueSchema("Input Value", "jsonPath"),
 });
 
 const feastFeaturesSchema = yup.object().shape({
@@ -113,7 +128,7 @@ export const feastInputSchema = yup.object().shape({
 const variableInputSchema = yup.object().shape({
   name: yup.string().required("Name is required"),
   type: yup.string().required("Type is required"),
-  value: yup.string().required("Value is required"),
+  value: stringValueSchema("Value", "jsonPath"),
 });
 
 const tablesInputSchema = yup.object().shape({