Fix: Display signal names instead of exit codes in worker termination logs

[ryuichi1208]

Fix signal name display in worker termination logs

Previously, when a worker process was terminated by a signal like SIGABRT,the log would display "Worker (pid:X) was sent code 134!" instead of the actual signal name. This happened because the code incorrectly interpreted shell exit codes (128 + signal_number) as signal numbers.

Changes:

• Use os.WIFSIGNALED() and os.WTERMSIG() to correctly detect and extract
  signal numbers from process exit status
• Use os.WIFEXITED() and os.WEXITSTATUS() to correctly extract exit codes
• Handle shell convention where exit code > 128 indicates signal termination
  (e.g., 134 = 128 + SIGABRT)
• Add tests for both direct signal termination and shell-style exit codes

After this fix, the log correctly displays:
"Worker (pid:X) was sent SIGABRT!" instead of "Worker (pid:X) was sent code 134!"

Fixes signal name logging for all signals including SIGABRT, SIGKILL, SIGTERM, etc.

issue: #3434