feat: add image download and save support to ImageSDK

[NianJiuZst]

Summary

Closes #144

Adds ImageSDK.save() — a method that downloads or decodes generated images to disk, matching the CLI's --out / --out-dir / --out-prefix behavior.

Motivation

ImageSDK.generate() returns URLs or base64 data, but leaves downloading and file I/O entirely to the user. Every SDK user who wants to save images must:

1. Manually download from CDN URLs (with retries for reliability)
2. Handle base64 decoding if using that response format
3. Create output directories
4. Implement filename generation

This is boilerplate that the CLI already handles. The SDK should too.

Usage

import { MiniMaxSDK } from 'mmx-cli/sdk';

const sdk = new MiniMaxSDK({ apiKey: 'sk-...' });

const result = await sdk.image.generate({
  prompt: 'A cat in a spacesuit',
  n: 3,
});

// Save to individual files in a directory
const paths = await sdk.image.save(result, {
  outDir: './generated',
  prefix: 'cat',
});
// → ['./generated/cat_001.jpg', './generated/cat_002.jpg', './generated/cat_003.jpg']

// Or save a single image to an exact path
await sdk.image.save(result, { out: '/tmp/avatar.png' });

Files changed

File	Change
src/sdk/image/index.ts	+51 lines — ImageSaveOptions interface + save() method
test/sdk/image.test.ts	+85 lines — 4 test cases for base64 save, multi-file, out conflict

Screenshots