fix(screencast): await pending auto-save in page.close()

[Git-Raini]

Follow-up to review comment on microsoft#41744 (comment)

The auto-save triggered from the once(Events.Page.Close, ...) listener was fire-and-forget - artifact.saveAs(savePath).catch(() => {}) with nothing awaiting it. That meant page.close() could resolve before the video was actually on disk, and a failed save produced no signal at all.

Screencast now tracks the in-flight save as _pendingAutoSave, and Page.close() awaits it in a finally block. This is safe: I traced PageDispatcher and confirmed the server dispatches the 'close' event (which triggers the auto-save) before it sends the response to the close() call itself - so _pendingAutoSave is always already assigned by the time page.close() reaches its finally block, regardless of which close path was taken (_ownedContext.close(), runBeforeUnload, or direct _channel.close()).

Diff:

--- a/packages/playwright-core/src/client/screencast.ts
+++ b/packages/playwright-core/src/client/screencast.ts
@@ private _artifact: Artifact | undefined;
+  private _pendingAutoSave: Promise<void> | undefined;
 ...
-        artifact.saveAs(savePath).catch(() => {});
+        this._pendingAutoSave = artifact.saveAs(savePath).catch(() => {});
 ...
+  async _waitForPendingAutoSave(): Promise<void> {
+    await this._pendingAutoSave;
+  }

--- a/packages/playwright-core/src/client/page.ts
+++ b/packages/playwright-core/src/client/page.ts
@@ close() { ... } catch (e) { ... }
+    } finally {
+      await this.screencast._waitForPendingAutoSave();
+    }

Test results - screencast.spec.ts, 195 runs (13 tests × 15 repeats), all passing

[182/195] [chromium-library] › tests\library\screencast.spec.ts:237:5 › start dispose stops recording
[183/195] [chromium-library] › tests\library\screencast.spec.ts:28:5 › screencast.start delivers frames via onFrame callback
[184/195] [chromium-library] › tests\library\screencast.spec.ts:57:5 › onFrame receives viewport size
[185/195] [chromium-library] › tests\library\screencast.spec.ts:108:5 › start returns a disposable that stops screencast
[186/195] [chromium-library] › tests\library\screencast.spec.ts:83:5 › start throws if screencast is already started
[187/195] [chromium-library] › tests\library\screencast.spec.ts:131:5 › start/stop twice without path creates two files in artifactsDir
[188/195] [chromium-library] › tests\library\screencast.spec.ts:94:5 › start allows restart with different options after stop
[189/195] [chromium-library] › tests\library\screencast.spec.ts:156:5 › start should work when recordVideo is set
[190/195] [chromium-library] › tests\library\screencast.spec.ts:179:5 › start should fail when another recording is in progress
[191/195] [chromium-library] › tests\library\screencast.spec.ts:186:5 › stop should not fail when no recording is in progress
[192/195] [chromium-library] › tests\library\screencast.spec.ts:194:5 › start should finish when page is closed
[193/195] [chromium-library] › tests\library\screencast.spec.ts:207:5 › should auto-save video when page closes without stop
[194/195] [chromium-library] › tests\library\screencast.spec.ts:225:5 › empty video
[195/195] [chromium-library] › tests\library\screencast.spec.ts:237:5 › start dispose stops recording
  195 passed (3.7m)

Test results - full page-close.spec.ts suite (18 tests), no regressions

[5/18] [chromium-library] › tests\library\page-close.spec.ts:65:5 › should set the page close state
[6/18] [chromium-library] › tests\library\page-close.spec.ts:71:5 › should pass page to close event
[7/18] [chromium-library] › tests\library\page-close.spec.ts:79:5 › should terminate network waiters
[8/18] [chromium-library] › tests\library\page-close.spec.ts:100:5 › should return null if parent page has been closed
[9/18] [chromium-library] › tests\library\page-close.spec.ts:92:5 › should be callable twice
[10/18] [chromium-library] › tests\library\page-close.spec.ts:110:5 › should fail with error upon disconnect
[11/18] [chromium-library] › tests\library\page-close.spec.ts:118:5 › page.close should work with window.close
[12/18] [chromium-library] › tests\library\page-close.spec.ts:124:5 › should not throw UnhandledPromiseRejection when page closes
[13/18] [chromium-library] › tests\library\page-close.spec.ts:133:5 › interrupt request.response() and request.allHeaders() on page.close
[14/18] [chromium-library] › tests\library\page-close.spec.ts:152:5 › should not treat navigations as new popups
[15/18] [chromium-library] › tests\library\page-close.spec.ts:166:5 › should not result in unhandled rejection
[16/18] [chromium-library] › tests\library\page-close.spec.ts:180:5 › should reject response.finished if page closes
[17/18] [chromium-library] › tests\library\page-close.spec.ts:200:5 › should not throw when continuing while page is closing
[18/18] [chromium-library] › tests\library\page-close.spec.ts:212:5 › should not throw when continuing after page is closed
  18 passed (3.3s)

Also confirmed tsc --noEmit and eslint clean on both changed files.