[api-minor] Attempt to support fetching the raw data of the PDF document from the PDFDocumentLoadingTask-instance (issue 15085)

The new API-functionality will allow a PDF document to be downloaded in the viewer e.g. while the PasswordPrompt is open, or in cases when document initialization failed.
Normally the raw data of the PDF document would be accessed via the `PDFDocumentProxy.prototype.getData` method, however in these cases the `PDFDocumentProxy`-instance isn't available.

test/unit/api_spec.js

@@ -828,6 +828,47 @@ describe("api", function () {
 
       await loadingTask.destroy();
     });
+
+    it("gets data, on failure, from `PDFDocumentLoadingTask`-instance", async function () {
+      const typedArrayPdf = await DefaultFileReaderFactory.fetch({
+        path: TEST_PDFS_PATH + "issue6010_1.pdf",
+      });
+
+      // Sanity check to make sure that we fetched the entire PDF file.
+      expect(typedArrayPdf instanceof Uint8Array).toEqual(true);
+      expect(typedArrayPdf.length).toEqual(1116);
+
+      const loadingTask = getDocument(typedArrayPdf.slice());
+      expect(loadingTask instanceof PDFDocumentLoadingTask).toEqual(true);
+
+      let passwordData = null;
+      // Attach the callback that is used to request a password;
+      // similarly to how the default viewer handles passwords.
+      loadingTask.onPassword = async (updatePassword, reason) => {
+        passwordData = await loadingTask.getData();
+
+        updatePassword(new Error("Should reject the loadingTask."));
+      };
+
+      try {
+        await loadingTask.promise;
+
+        // Shouldn't get here.
+        expect(false).toEqual(true);
+      } catch (ex) {
+        expect(ex instanceof PasswordException).toEqual(true);
+        expect(ex.code).toEqual(PasswordResponses.NEED_PASSWORD);
+      }
+
+      // Ensure that the raw PDF document can be fetched while
+      // an `onPassword` callback is delaying initialization...
+      expect(passwordData).toEqual(typedArrayPdf);
+      // ... and once an exception has stopped initialization.
+      const data = await loadingTask.getData();
+      expect(data).toEqual(typedArrayPdf);
+
+      await loadingTask.destroy();
+    });
   });
 
   describe("PDFWorker", function () {