Auto-create log directory for telemetry

[SmartManoj]

Problem

When log_completions=True is set, logging silently fails if the directory doesn't exist:

• Users see a UserWarning at the end of the output.
• All logs for the entire session are lost

llm = LLM(model="gpt-4", log_completions=True)
# UserWarning: Telemetry logging failed: log_dir does not exist: logs/completions

Solution

Auto-create the log directory.

Why This Approach?

1. Better UX: Users expect log_completions=True to work out of the box
2. Fail fast: Permission errors still raise immediately with clear messages
3. Standard practice: SDKs routinely create directories (cache dirs, config dirs, etc.)
4. No data loss: Previous behavior silently dropped all logs

[blacksmith-sh]

[Automatic Post]: I have assigned @CLFutureX as a reviewer based on git blame information. Thanks in advance for the help!

[xingyaoww]

Thanks