fix(storage): correct presign content-type claim; head returns contentType

Spec review (live negative test) found s3-request-presigner unconditionally
unsigns content-type: header-less PUT returns 200, so the previous docblock
claiming signature enforcement was a false guarantee. Content-Type is now
documented as convention-only; head() returns contentType so the complete
step (Task 19) can verify it server-side against the session declaration.

Co-Authored-By: Claude Code <noreply@anthropic.com>

packages/storage/test/minio.adapter.spec.ts

@@ -31,6 +31,7 @@ describe('MinioStorageAdapter（真实 MinIO 集成）', () => {
     expect(head).not.toBeNull();
     expect(head?.bytes).toBe(body.length);
     expect(head?.etag).toBeTruthy();
+    expect(head?.contentType).toBe('application/octet-stream');
   });
 
   it('put + getStream: 写入后流式读回的字节与写入一致', async () => {