Make nob_default_handler work in multi-threaded or multi-process

[ChenXinyu-CHD]

The old implementation of nob_default_handler works well in a single‑threaded environment, but it may produce unexpected output in a multi‑process environment. For example, the following output is generated in my project:

[ERROR] [ERROR] [ERROR] Could not read file ../tests/if_else.mus.out: No such file or directoryCould not read file ../tests/invoke_extern.mus.out: No such file or directoryCould not read file ../tests/binop.mus.out: No such file or directory

This happens because nob_default_handler invokes fprintf multiple times, and another process may print something else between the multiple fprintf calls inside nob_default_handler.

To fix this issue, I use nob_temp_svprintf to generate the complete message first, and then print it with a single fprintf call.