Fix listing and download completions cli bug

[kxtran]

Description

• Keep file open while writing file to it improves the latency.
• Fix the bugs with listing completions and downloading completions.

Opening file each time duration:

Downloaded 10 completions so far to completions.jsonl.
Downloaded 20 completions so far to completions.jsonl.
Download total completions: 20. Saved to completions.jsonl
Total write time: 9985.4531 ms

Keep the file open:

Downloaded 10 completions so far to completions.jsonl.
Downloaded 20 completions so far to completions.jsonl.
Download total completions: 20. Saved to completions.jsonl
Total write time: 1.5178 ms

Screen.Recording.2024-07-31.at.18.59.42.mov