fix(s3): use Buffer instead of string for health check body (#6913)

When using ali-oss as STORAGE_VENDOR, the `put()` method treats string
  body as a file path and calls `fs.statSync()` on it. Passing `'ok'`
  causes `ENOENT: no such file or directory, stat 'ok'` during
  instrumentation-check, crashing the server on startup.

  Using `Buffer.from('ok')` ensures all adapters (aws-s3, minio, cos, oss)
  handle the body as content rather than a file path.

packages/service/common/s3/buckets/base.ts

@@ -73,7 +73,7 @@ export class S3BaseBucket {
 
     await this.client.uploadObject({
       key,
-      body: 'ok',
+      body: Buffer.from('ok'),
       contentType: 'text/plain',
       metadata: {
         contentDisposition: getContentDisposition({ filename, type: 'attachment' }),