refactor(storage-resize-images): use Node built-ins for uuid and mkdir

[IzaakGough]

Stacked on #2946. Retarget to kits once that merges.

What was broken: the kit depends on mkdirp and uuid for things Node covers natively. The extension removed both, along with uuidv4 and rimraf, in 9bb0c1fa and c6fbf3b7, which landed before the kit was created.

What changed: six call sites move to crypto.randomUUID() and fs.promises.mkdir(dir, { recursive: true }), matching the extension. Both dependencies come out of package.json. mkdirp leaves the tree entirely; uuid remains transitively via genkit and the Google client libraries, so this is dependency hygiene rather than a fix for the uuid advisory.

Verification: npm ci and npm run build are clean. Exercised the changed paths directly: downloadOriginalFile writes to a uuid-named file under the temp dir with contents intact, two consecutive calls produce distinct paths and neither fails on the already-existing directory, replaceWithDefaultPlaceholder swaps in the bundled placeholder, and constructMetadata regenerates firebaseStorageDownloadTokens as a uuid when REGENERATE_TOKEN is on. The full Storage upload path was not exercised.

[gemini-code-assist]

Code Review

This pull request removes the external dependencies mkdirp and uuid from the storage-resize-images kit, replacing them with native Node.js APIs (fs.promises.mkdir and crypto.randomUUID()). It also simplifies a TypeScript type assertion in util.ts. There are no review comments, so I have no feedback to provide.