* Fix _kill_process AttributeError when _stats_logger is unset
LlamaCppBackend._kill_process references self._stats_logger in its finally
block, but __init__ only sets self._stats_logger = None partway through. If
__init__ raises before that line, or the backend is built via __new__ (as the
kill-path unit test does), teardown crashes with AttributeError instead of
cleaning up the process.
Guard the reference with getattr, matching the existing
hasattr(self, '_chat_template_file') guard in the same finally block. Fixes
test_kill_process_records_timestamp_on_actual_kill.
* Also guard _stdout_thread in _kill_process teardown
Review follow-up: the same finally block also reads self._stdout_thread, which
is unset on a partially-built / __new__ backend. Guard it with getattr like
_llama_log_fh below, and add a test that _kill_process tolerates a backend with
those optional attrs unset. Trim the _stats_logger comment.
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Co-authored-by: Michael Han <michaelhan2050@gmail.com>
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