tools : use common_log_pause to fix fit-params output race

[Aadeshveer]

Replaced the std::this_thread::sleep_for(10ms) hack with common_log_pause(common_log_main()) in llama-fit-params.

Reasoning:
The previous 10ms delay was intended to let the async logger drain its queue to stderr before the main thread printed the result to stdout. However, on high-performance or loaded systems, 10ms is not guaranteed to be enough, leading to interleaved output (see Issue #18085).

common_log_pause explicitly waits for the background worker to drain the log queue, ensuring deterministic separation between logs and the final result.

Fixes #18085

[ngxson]

you should use log_flush instead

[Aadeshveer]

changed common_log_pause to common_log_flush