test(import): read the upload body instead of parsing it as FormData

These are the only two test files that call request.formData() inside an MSW
handler, and they hold exactly the three tests that failed on CI while passing
locally. The body axios' XHR adapter produces under jsdom is one Node 24's
multipart parser rejects, so the handler threw before reading a field and the
component reported a failed upload. Reading the raw payload asserts the same
thing without depending on the parser.

client/tests/helpers/multipart.ts

@@ -0,0 +1,42 @@
+/**
+ * Reads the fields of a multipart request body without going through
+ * `Request.formData()`.
+ *
+ * Uploads in this app go out through axios' XHR adapter, and the body jsdom
+ * produces for that is one Node's multipart parser refuses on the CI runner
+ * (Node 24) while accepting it locally (Node 22) — the handler then throws
+ * before it ever sees a field. Reading the raw payload keeps the assertion on
+ * what was actually sent and takes the parser out of the picture.
+ *
+ * The body is a stream and can only be read once, so this returns everything in
+ * one pass. File parts are reported by filename rather than by content.
+ */
+export interface MultipartBody {
+  /** Plain text fields, by name. */
+  fields: Record<string, string>;
+  /** Names of the uploaded files, in the order they were appended. */
+  filenames: string[];
+}
+
+export async function readMultipart(request: Request): Promise<MultipartBody> {
+  const body = await request.text();
+  const boundary = request.headers.get('content-type')?.match(/boundary=(?:"([^"]+)"|([^;]+))/i);
+  const marker = boundary ? `--${(boundary[1] ?? boundary[2]).trim()}` : null;
+
+  const fields: Record<string, string> = {};
+  const filenames: string[] = [];
+
+  // Without a boundary there is nothing reliable to split on — hand back an
+  // empty result so the assertion fails on the value rather than on a throw.
+  if (!marker) return { fields, filenames };
+
+  for (const part of body.split(marker).slice(1)) {
+    const name = part.match(/name="([^"]*)"/)?.[1];
+    if (!name) continue;
+    const filename = part.match(/filename="([^"]*)"/)?.[1];
+    if (filename !== undefined) { filenames.push(filename); continue; }
+    // Headers, blank line, then the value up to the trailing CRLF.
+    fields[name] = part.match(/\r?\n\r?\n([\s\S]*?)\r?\n?$/)?.[1] ?? '';
+  }
+  return { fields, filenames };
+}