fix: protect downloads and stream JSON output

[NianJiuZst]

Summary

• Write downloads to a temporary file and atomically replace the destination only after a complete transfer.
• Keep mmx text chat --output json machine-readable by avoiding default streaming and suppressing token writes for explicit streamed JSON.
• Add regression tests for failed downloads and streamed JSON output.

Testing

• bun test test/files/download.test.ts test/commands/text/chat.test.ts test/commands/video/download.test.ts test/commands/video/generate.test.ts
• bun run typecheck