Preserve DontEnum on EventSource reassignment, fix test leak

Changes:
- Preserve DontEnum attribute when EventSource is reassigned so the
  property remains non-enumerable after reassignment
- Wrap EventSource reassignment test in try/finally to ensure the
  original EventSource is always restored even if assertions fail

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/bun.js/bindings/ZigGlobalObject.cpp

@@ -937,8 +937,8 @@ JSC_DEFINE_CUSTOM_SETTER(EventSource_setter,
     JSValue value = JSValue::decode(encodedValue);
     auto* globalObject = jsCast<Zig::GlobalObject*>(lexicalGlobalObject);
 
-    // Replace the accessor with a plain data property
-    globalObject->putDirect(vm, Identifier::fromString(vm, "EventSource"_s), value, 0);
+    // Replace the accessor with a plain data property, preserving DontEnum
+    globalObject->putDirect(vm, Identifier::fromString(vm, "EventSource"_s), value, static_cast<unsigned>(PropertyAttribute::DontEnum));
     return true;
 }
 

test/regression/issue/3319.test.ts

@@ -546,12 +546,14 @@ describe("EventSource", () => {
       const original = EventSource;
       const fake = function FakeEventSource() {};
 
-      // Reassign should work
-      (globalThis as any).EventSource = fake;
-      expect(EventSource).toBe(fake);
-
-      // Restore
-      (globalThis as any).EventSource = original;
+      try {
+        // Reassign should work
+        (globalThis as any).EventSource = fake;
+        expect(EventSource).toBe(fake);
+      } finally {
+        // Always restore original to avoid leaking mutated global
+        (globalThis as any).EventSource = original;
+      }
       expect(EventSource).toBe(original);
     });
   });