test(frontend): add spec for BlobErrorHttpInterceptor (#5461)

### What changes were proposed in this PR?

Adds a unit spec for `BlobErrorHttpInterceptor`, which previously had
none. Covers every branch of `intercept()`:

- success → passed through unchanged
- re-thrown unchanged for: a non-`HttpErrorResponse` error, an
`HttpErrorResponse` whose `error` is not a `Blob`, and a `Blob` error
whose type is not `application/json`
- an `application/json` `Blob` error → parsed into a structured
`HttpErrorResponse` with the original `status` / `statusText` / `url`
preserved
- malformed JSON in the blob, or a `FileReader` failure → falls back to
the original error

The interceptor is driven directly with a stub `HttpHandler` rather than
through `HttpClient`/`HTTP_INTERCEPTORS`. Two branches — a
non-`HttpErrorResponse` error and a `FileReader` failure — cannot be
produced through `HttpClient` (it always wraps errors as
`HttpErrorResponse`, and a readable `Blob` never triggers
`FileReader.onerror`), so direct invocation is the only way to cover
them. Follows `frontend/TESTING.md` (Vitest).

### Any related issues, documentation, discussions?

Closes #5455.

### How was this PR tested?

`yarn test --include='**/blob-error-http-interceptor.service.spec.ts'` →
7 passed. `prettier --check` clean.

### Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (claude-opus-4-7)

---------

Signed-off-by: Meng Wang <mengw15@uci.edu>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: mengw15

frontend/src/app/common/service/blob-error-http-interceptor.service.spec.ts

@@ -0,0 +1,143 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import {
+  HttpErrorResponse,
+  HttpEvent,
+  HttpHandler,
+  HttpHeaders,
+  HttpRequest,
+  HttpResponse,
+} from "@angular/common/http";
+import { Observable, firstValueFrom, of, throwError } from "rxjs";
+
+import { BlobErrorHttpInterceptor } from "./blob-error-http-interceptor.service";
+
+/**
+ * The interceptor is a pure function of (req, next), so the specs drive it
+ * directly with a stub `HttpHandler` rather than through HttpClient. Two of
+ * the branches under test — a non-`HttpErrorResponse` error and a
+ * `FileReader` failure — cannot be produced through `HttpClient` at all
+ * (it always wraps errors as `HttpErrorResponse`, and a readable Blob never
+ * triggers `FileReader.onerror`), so direct invocation is the only way to
+ * cover them.
+ */
+describe("BlobErrorHttpInterceptor", () => {
+  let interceptor: BlobErrorHttpInterceptor;
+  const req = new HttpRequest("GET", "/test");
+
+  const handlerReturning = (obs: Observable<HttpEvent<any>>): HttpHandler => ({
+    handle: (_req: HttpRequest<any>) => obs,
+  });
+
+  // Run the interceptor and resolve to the emitted value or, on error, the error.
+  const run = (next: HttpHandler): Promise<any> => firstValueFrom(interceptor.intercept(req, next)).catch(e => e);
+
+  beforeEach(() => {
+    interceptor = new BlobErrorHttpInterceptor();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("passes a successful response through unchanged", async () => {
+    const response = new HttpResponse({ body: "ok", status: 200 });
+    expect(await run(handlerReturning(of(response)))).toBe(response);
+  });
+
+  it("re-throws an error that is not an HttpErrorResponse unchanged", async () => {
+    const err = new Error("not-http");
+    expect(await run(handlerReturning(throwError(() => err)))).toBe(err);
+  });
+
+  it("re-throws an HttpErrorResponse whose error is not a Blob unchanged", async () => {
+    const err = new HttpErrorResponse({ error: { message: "plain" }, status: 500 });
+    expect(await run(handlerReturning(throwError(() => err)))).toBe(err);
+  });
+
+  it("re-throws an HttpErrorResponse with a non-json Blob unchanged", async () => {
+    const err = new HttpErrorResponse({
+      error: new Blob(["whatever"], { type: "text/plain" }),
+      status: 500,
+    });
+    expect(await run(handlerReturning(throwError(() => err)))).toBe(err);
+  });
+
+  it("parses an application/json Blob error into a new HttpErrorResponse, preserving status/headers/url", async () => {
+    const err = new HttpErrorResponse({
+      error: new Blob([JSON.stringify({ message: "Boom" })], { type: "application/json" }),
+      status: 502,
+      statusText: "Bad Gateway",
+      url: "http://example.com/api",
+      headers: new HttpHeaders({ "x-request-id": "trace-123" }),
+    });
+
+    const rejected = await run(handlerReturning(throwError(() => err)));
+
+    expect(rejected).toBeInstanceOf(HttpErrorResponse);
+    expect(rejected).not.toBe(err); // a new instance was constructed, not the original
+    expect(rejected.error).toEqual({ message: "Boom" });
+    expect(rejected.status).toBe(502);
+    expect(rejected.statusText).toBe("Bad Gateway");
+    expect(rejected.url).toBe("http://example.com/api");
+    expect(rejected.headers.get("x-request-id")).toBe("trace-123");
+  });
+
+  it("builds a new error with a null url when the original error has no url", async () => {
+    const err = new HttpErrorResponse({
+      error: new Blob([JSON.stringify({ message: "Boom" })], { type: "application/json" }),
+      status: 500,
+      // url omitted → HttpErrorResponse defaults it to null, exercising the
+      // `err.url !== null ? err.url : undefined` false branch.
+    });
+
+    const rejected = await run(handlerReturning(throwError(() => err)));
+
+    expect(rejected).toBeInstanceOf(HttpErrorResponse);
+    expect(rejected).not.toBe(err); // a new instance was constructed, not the original
+    expect(rejected.error).toEqual({ message: "Boom" });
+    expect(rejected.url).toBeNull();
+  });
+
+  it("re-throws the original error when the Blob contains malformed JSON", async () => {
+    const err = new HttpErrorResponse({
+      error: new Blob(["not json {"], { type: "application/json" }),
+      status: 500,
+    });
+    expect(await run(handlerReturning(throwError(() => err)))).toBe(err);
+  });
+
+  it("re-throws the original error when the FileReader fails", async () => {
+    class FailingFileReader {
+      onload: ((e: Event) => void) | null = null;
+      onerror: ((e: Event) => void) | null = null;
+      readAsText(): void {
+        this.onerror?.(new Event("error"));
+      }
+    }
+    vi.stubGlobal("FileReader", FailingFileReader);
+
+    const err = new HttpErrorResponse({
+      error: new Blob([JSON.stringify({ message: "Boom" })], { type: "application/json" }),
+      status: 500,
+    });
+    expect(await run(handlerReturning(throwError(() => err)))).toBe(err);
+  });
+});