Return the path of the active debug log file, or None if not logging.
Reflects the file handler actually attached by configure_debug_logging,
not the current DEEPAGENTS_CODE_DEBUG env value. The two diverge when the
variable is set after import e.g. via a project/global .env loaded during
settings bootstrap in which case the variable reads truthy but no handler
was installed and no log file exists. Callers that surface "full error in
" hints must use this rather than the env var to avoid pointing users
at a file that was never created.