DEV: Allow ember hash as a context in applyValueTransformer (#29922)

This unlocks the ability to use that function directly in templates:

```hbs
{{applyValueTransformer
  "foo-bar"
  @defaultValue
  (hash arg1=@arg1 arg2=@arg2)
}}
```

app/assets/javascripts/discourse/app/lib/transformer.js

@@ -317,10 +317,13 @@ export function applyValueTransformer(
 
   if (
     typeof (context ?? undefined) !== "undefined" &&
-    !(typeof context === "object" && context.constructor === Object)
+    !(
+      typeof context === "object" &&
+      (context.constructor === Object || context.constructor === undefined)
+    )
   ) {
     throw (
-      `${prefix}("${transformerName}", ...): context must be a simple JS object or nullish.\n` +
+      `${prefix}("${transformerName}", ...): context must be a simple JS object/an Ember hash or nullish.\n` +
       "Avoid passing complex objects in the context, like for example, component instances or objects that carry " +
       "mutable state directly. This can induce users to registry transformers with callbacks causing side effects " +
       "and mutating the context directly. Inevitably, this leads to fragile integrations."

app/assets/javascripts/discourse/tests/unit/lib/transformer-test.js

@@ -14,6 +14,15 @@ import {
   transformerWasAdded,
 } from "discourse/lib/transformer";
 
+function notThrows(testCallback) {
+  try {
+    testCallback();
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 module("Unit | Utility | transformers", function (hooks) {
   setupTest(hooks);
 
@@ -208,15 +217,6 @@ module("Unit | Utility | transformers", function (hooks) {
     });
 
     test("accepts only simple objects as context", function (assert) {
-      const notThrows = (testCallback) => {
-        try {
-          testCallback();
-          return true;
-        } catch {
-          return false;
-        }
-      };
-
       assert.ok(
         notThrows(() =>
           applyValueTransformer("test-value1-transformer", "foo")
@@ -317,6 +317,20 @@ module("Unit | Utility | transformers", function (hooks) {
       );
     });
 
+    test("accepts an ember hash proxy as context", function (assert) {
+      // A hash-like object
+      const hash = { a: 1 };
+      Object.setPrototypeOf(hash, null);
+      const proxy = new Proxy(hash, {});
+
+      assert.true(
+        notThrows(() =>
+          applyValueTransformer("test-value1-transformer", "foo", proxy)
+        ),
+        "doesn't throw an error if context is a proxy to an ember hash object"
+      );
+    });
+
     test("applying the transformer works", function (assert) {
       class Testable {
         #value;
@@ -777,15 +791,6 @@ module("Unit | Utility | transformers", function (hooks) {
     });
 
     test("accepts only simple objects as context", function (assert) {
-      const notThrows = (testCallback) => {
-        try {
-          testCallback();
-          return true;
-        } catch {
-          return false;
-        }
-      };
-
       assert.ok(
         notThrows(() =>
           applyBehaviorTransformer("test-behavior1-transformer", () => true)