test: adds cypress spec to test onpage unload functionality (#41084)

## Description
Adds spec file and fixture file for on page unload functionality

Fixes #41000
_or_  
Fixes `Issue URL`
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags="@tag.JS, @tag.Sanity"

### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/16070917983>
> Commit: 9435c88d922a2c1344cc5feb955ce3eb5f8620f5
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=16070917983&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.JS, @tag.Sanity`
> Spec:
> <hr>Fri, 04 Jul 2025 10:45:13 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Added a comprehensive end-to-end test suite to verify on-page unload
behavior across different application modes and navigation methods.
* Included scenarios to ensure unload handlers trigger correctly and do
not fire redundantly.
  * Confirmed execution of multiple unload handlers in preview mode.

* **Chores**
* Introduced a new application fixture for testing on-page unload
actions with multiple pages, widgets, and JavaScript actions.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/client/cypress/e2e/Regression/ServerSide/OnLoadTests/OnPageUnload_spec.ts

@@ -0,0 +1,112 @@
+// Import necessary helpers and libraries.
+import { featureFlagIntercept } from "../../../../support/Objects/FeatureFlags";
+import {
+  agHelper,
+  appSettings,
+  assertHelper,
+  deployMode,
+  homePage,
+  locators,
+} from "../../../../support/Objects/ObjectsCore";
+import EditorNavigation, {
+  EntityType,
+} from "../../../../support/Pages/EditorNavigation";
+import pageList from "../../../../support/Pages/PageList";
+
+describe(
+  "On-Page Unload Functionality",
+  { tags: ["@tag.JS", "@tag.Sanity"] },
+  () => {
+    const page1 = "Page1 - with long long name";
+    const page2 = "Page2 - with long long name";
+    const page3 = "Page3 - with long long name";
+    const page1ToastForOnPageUnload = "Page 1 on page unload.";
+    const page2ToastForOnPageUnload = "Page 2 on page unload.";
+    const page1ButtonText = "Submit";
+    // Setup: Runs once before all tests in this block.
+    before(() => {
+      homePage.NavigateToHome();
+      homePage.ImportApp("onPageUnloadBehavior_app.json");
+      assertHelper.AssertNetworkStatus("@importNewApplication");
+      featureFlagIntercept({
+        release_jsobjects_onpageunloadactions_enabled: true,
+      });
+    });
+
+    it("1. [Deployed Mode] Nav via links: Triggers unload, then doesn't on return", () => {
+      deployMode.DeployApp();
+      // Start on Page 1 and navigate to Page 2.
+      agHelper.WaitUntilEleAppear(appSettings.locators._header);
+      agHelper.AssertElementVisibility(
+        appSettings.locators._getActivePage(page1),
+      );
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page2,
+      );
+      agHelper.ValidateToastMessage(page1ToastForOnPageUnload);
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page1,
+      );
+      deployMode.NavigateBacktoEditor();
+    });
+
+    it("2. [Edit Mode] Nav via Page Selector: Triggers unload", () => {
+      EditorNavigation.SelectEntityByName(page2, EntityType.Page);
+      agHelper.ValidateToastMessage(page1ToastForOnPageUnload);
+      agHelper.WaitUntilAllToastsDisappear();
+
+      EditorNavigation.SelectEntityByName(page3, EntityType.Page);
+      agHelper.ValidateToastMessage(page2ToastForOnPageUnload);
+      agHelper.WaitUntilAllToastsDisappear();
+
+      EditorNavigation.SelectEntityByName(page1, EntityType.Page);
+    });
+
+    it("3. [Preview mode] Multiple Handlers: Triggers all handlers", () => {
+      agHelper.GetNClick(locators._enterPreviewMode);
+      agHelper.AssertElementVisibility(
+        appSettings.locators._getActivePage(page1),
+      );
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page2,
+      );
+      agHelper.ValidateToastMessage(page1ToastForOnPageUnload);
+      agHelper.WaitUntilAllToastsDisappear();
+
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page3,
+      );
+      agHelper.ValidateToastMessage(page2ToastForOnPageUnload);
+      agHelper.WaitUntilAllToastsDisappear();
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page1,
+      );
+      agHelper.GetNClick(locators._exitPreviewMode);
+    });
+
+    it("4. [Both Modes - Programmatic nav]: Triggers unload via button click", () => {
+      agHelper.ClickButton(page1ButtonText);
+      agHelper.ValidateToastMessage(page1ToastForOnPageUnload);
+      agHelper.WaitUntilAllToastsDisappear();
+      pageList.ShowList();
+      EditorNavigation.SelectEntityByName(page1, EntityType.Page);
+
+      deployMode.DeployApp();
+      agHelper.ClickButton(page1ButtonText);
+      agHelper.ValidateToastMessage(page1ToastForOnPageUnload);
+      agHelper.AssertElementVisibility(
+        appSettings.locators._getActivePage(page2),
+      );
+      agHelper.GetNClickByContains(
+        appSettings.locators._navigationMenuItem,
+        page1,
+      );
+      deployMode.NavigateBacktoEditor();
+    });
+  },
+);