test: add Playwright integration tests for PostBack demo (Phase 2)

- PostBack button trigger test
- PostBack hyperlink click test
- ScriptManager startup script registration test
- CI-friendly timeouts (30s) learned from Phase 1

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz

samples/AfterBlazorServerSide.Tests/Migration/PostBackTests.cs

@@ -0,0 +1,154 @@
+using Microsoft.Playwright;
+
+namespace AfterBlazorServerSide.Tests.Migration;
+
+/// <summary>
+/// Integration tests for the PostBack demo page at /postback-demo.
+/// Verifies GetPostBackEventReference, GetPostBackClientHyperlink,
+/// and ScriptManager.GetCurrent startup script registration.
+/// </summary>
+[Collection(nameof(PlaywrightCollection))]
+public class PostBackTests
+{
+    private readonly PlaywrightFixture _fixture;
+
+    public PostBackTests(PlaywrightFixture fixture)
+    {
+        _fixture = fixture;
+    }
+
+    /// <summary>
+    /// Verifies that clicking the postback button triggers a __doPostBack call
+    /// and the result area displays the event target and argument.
+    /// </summary>
+    [Fact]
+    public async Task PostBack_Button_TriggersPostBackEvent()
+    {
+        var page = await _fixture.NewPageAsync();
+
+        try
+        {
+            await page.GotoAsync($"{_fixture.BaseUrl}/postback-demo", new PageGotoOptions
+            {
+                WaitUntil = WaitUntilState.NetworkIdle,
+                Timeout = 30000
+            });
+
+            // Wait for Blazor interactive circuit to be ready
+            var button = page.Locator("#postback-button");
+            await button.WaitForAsync(new LocatorWaitForOptions
+            {
+                State = WaitForSelectorState.Attached,
+                Timeout = 10000
+            });
+
+            await button.ClickAsync();
+
+            // Wait for the result to appear after the server round-trip
+            var result = page.Locator("#postback-result");
+            await result.WaitForAsync(new LocatorWaitForOptions
+            {
+                State = WaitForSelectorState.Attached,
+                Timeout = 10000
+            });
+
+            // Allow time for Blazor to process the postback and re-render
+            await page.WaitForTimeoutAsync(2000);
+
+            var resultText = await result.TextContentAsync();
+            Assert.NotNull(resultText);
+            Assert.Contains("PostBack received!", resultText!);
+        }
+        finally
+        {
+            await page.CloseAsync();
+        }
+    }
+
+    /// <summary>
+    /// Verifies that clicking the postback hyperlink (javascript:__doPostBack)
+    /// triggers a postback and the hyperlink result area is updated.
+    /// </summary>
+    [Fact]
+    public async Task PostBackHyperlink_Click_TriggersPostBackEvent()
+    {
+        var page = await _fixture.NewPageAsync();
+
+        try
+        {
+            await page.GotoAsync($"{_fixture.BaseUrl}/postback-demo", new PageGotoOptions
+            {
+                WaitUntil = WaitUntilState.NetworkIdle,
+                Timeout = 30000
+            });
+
+            // Wait for Blazor interactive circuit to be ready
+            var link = page.Locator("#postback-link");
+            await link.WaitForAsync(new LocatorWaitForOptions
+            {
+                State = WaitForSelectorState.Attached,
+                Timeout = 10000
+            });
+
+            await link.ClickAsync();
+
+            // Wait for the result to appear after the server round-trip
+            var result = page.Locator("#hyperlink-result");
+            await result.WaitForAsync(new LocatorWaitForOptions
+            {
+                State = WaitForSelectorState.Attached,
+                Timeout = 10000
+            });
+
+            // Allow time for Blazor to process the postback and re-render
+            await page.WaitForTimeoutAsync(2000);
+
+            var resultText = await result.TextContentAsync();
+            Assert.NotNull(resultText);
+            Assert.Contains("PostBack received!", resultText!);
+        }
+        finally
+        {
+            await page.CloseAsync();
+        }
+    }
+
+    /// <summary>
+    /// Verifies that ScriptManager.GetCurrent registers a startup script
+    /// that populates the target div on page load.
+    /// </summary>
+    [Fact]
+    public async Task ScriptManager_GetCurrent_RegistersStartupScript()
+    {
+        var page = await _fixture.NewPageAsync();
+
+        try
+        {
+            await page.GotoAsync($"{_fixture.BaseUrl}/postback-demo", new PageGotoOptions
+            {
+                WaitUntil = WaitUntilState.NetworkIdle,
+                Timeout = 30000
+            });
+
+            // Wait for the startup script to execute and populate the target
+            var target = page.Locator("#scriptmanager-target");
+            await target.WaitForAsync(new LocatorWaitForOptions
+            {
+                State = WaitForSelectorState.Attached,
+                Timeout = 10000
+            });
+
+            // Allow time for OnAfterRenderAsync to fire and JS to execute
+            await page.WaitForTimeoutAsync(2000);
+
+            var targetText = await target.TextContentAsync();
+            Assert.NotNull(targetText);
+            Assert.False(string.IsNullOrWhiteSpace(targetText),
+                "ScriptManager target should have text content after startup script runs");
+        }
+        finally
+        {
+            await page.CloseAsync();
+        }
+    }
+}