Remove form-data from cli-kit/app

[amcaplan]

Remove form-data from @shopify/cli-kit

Why: Part of an initiative to cut low-value dependency churn (14 Dependabot bumps / 24 months).

Replacement: Native FormData (Node ≥ 22.12) + structural typing.

• api.ts: replaced the form-data Headers type import with a structural type.
• bundle.ts (GCS upload): kept as a raw PUT with a manually-set Content-Type boundary — wire behavior is preserved (verified against existing upload tests).

⚠️ Reviewer callouts (MEDIUM confidence — review carefully):

• The public formData() helper now returns a native FormData, which drops .getHeaders() and .getBoundary() that the form-data instance exposed. This is a public-surface change — please confirm no external consumer relies on those methods.
• This branch was implemented partly via a fallback agent path (the primary launch hit a transient infra timeout); the diff scope has been verified to touch only the expected files.

Validation: type-check ✅, lint ✅, vitest ✅ (93 tests, including deploy/upload and dev-session-process).

🤖 AI-generated draft — needs human review before merge.

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/private/node/api.d.ts

@@ -1,4 +1,7 @@
-import { Headers } from 'form-data';
+export interface AnyHeaders {
+    forEach: (callback: (value: any, key: any) => void) => void;
+    get?: (name: string) => string | null;
+}
 export type API = 'admin' | 'storefront-renderer' | 'partners' | 'business-platform' | 'app-management';
 export declare const allAPIs: API[];
 export type NetworkRetryBehaviour = {
@@ -38,7 +41,7 @@ export declare function isTransientNetworkError(error: unknown): boolean;
  */
 export declare function isNetworkError(error: unknown): boolean;
 export declare function simpleRequestWithDebugLog<T extends {
-    headers: Headers;
+    headers: AnyHeaders;
     status: number;
 }>(requestOptions: RequestOptions<T>, errorHandler?: (error: unknown, requestId: string | undefined) => unknown): Promise<T>;
 /**
@@ -59,7 +62,7 @@ export declare function simpleRequestWithDebugLog<T extends {
  * @returns The response from the request
  */
 export declare function retryAwareRequest<T extends {
-    headers: Headers;
+    headers: AnyHeaders;
     status: number;
 }>(requestOptions: RequestOptions<T>, errorHandler?: (error: unknown, requestId: string | undefined) => unknown, retryOptions?: {
     limitRetriesTo?: number;

packages/cli-kit/dist/public/node/http.d.ts

@@ -1,5 +1,4 @@
 import { NetworkRetryBehaviour } from '../../private/node/api.js';
-import FormData from 'form-data';
 import { RequestInfo, RequestInit, Response } from 'node-fetch';
 export { FetchError, Request, Response } from 'node-fetch';
 /**