Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@tests/integration/defs/perf/utils.py`:
- Around line 622-624: The CalledProcessError handling calls e.stdout.decode()
(and elsewhere e.stderr.decode()) without defensive decoding; change decoding to
use e.stdout.decode("utf-8", errors="replace") and likewise
e.stderr.decode("utf-8", errors="replace") so UnicodeDecodeError won't mask the
real failure, ensuring you still pass the decoded string into clean_myelin_time
and any stderr handling in the CalledProcessError branch.
- Around line 162-165: The subprocess launcher in utils.py spawns MPI worker
processes that survive proc.kill(); modify the subprocess.Popen call that
creates proc (the one building benchmark_cmd / mpi_cmd) to include
start_new_session=True and add import signal to the module imports, then replace
all proc.kill() calls that attempt to terminate the stalled benchmark (the ones
referencing proc at the stall/cleanup paths) with os.killpg(proc.pid,
signal.SIGKILL) so the entire process group (MPI launcher + worker ranks) is
killed; ensure you import signal (and confirm os is already imported) and update
the three places where proc is terminated accordingly.
- Around line 226-240: The except block currently catches BaseException and
converts control-flow signals into a CalledProcessError; change it to first
catch Exception to perform the cleanup, conversion and raise CalledProcessError
(referencing proc, thread, output_lines, lock, cmd,
subprocess.CalledProcessError), and then add a second except BaseException:
handler that does the same cleanup (kill proc, wait, join thread, grab partial
under lock) but does NOT convert or wrap the exception—simply re-raise the
original exc to preserve interrupts and test-harness signals.

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Nitpick comments:
In `@tests/integration/defs/perf/utils.py`:
- Line 145: Annotate the new helper _run_command_with_captured_output by adding
type hints: set cmd: list[str], env: dict[str, str] | None (or None by default),
and the return type to -> str; update the function signature to def
_run_command_with_captured_output(cmd: list[str], env: dict[str, str] | None =
None) -> str so the subprocess path remains type-checkable and uses built-in
collection types.