fix: report stderr as None when it is merged into stdout

[LeMollinger]

Problem

execute(handle_errors=False) returns a CompletedProcess whose .stderr is always empty, whatever the command wrote. Everything the command printed is in .stdout.

CHILD = "import sys; sys.stderr.write('boom'); sys.exit(1)"
result = SubprocessExecutor(["python", "-c", CHILD]).execute(handle_errors=False)
print(repr(result.returncode), repr(result.stderr), repr(result.stdout))
# before: 1 '' 'boom'
# after:  1 None 'boom'

"" claims the command wrote nothing to stderr, so code like logger.warning(f"failed:\n{result.stderr}") reads as correct and stays permanently blank.

Why this fix

The streams are merged on purpose (stderr=subprocess.STDOUT), which keeps the progress and error lines in chronological order. That behaviour is right and stays.

Only the reported value is wrong. There is no second pipe to read, so stderr keeps its initial "". Setting that initial value to None fixes it: None is what the standard library means by "not captured", and it is what the print_output=False path already returned via communicate() — the three execution paths simply disagreed with each other.

The docstrings now say that stderr is merged into stdout and callers read .stdout.

[LeMollinger]

The second commit is unrelated to the fix: pypeline-runner now rejects an unquoted python_version in pypeline.yaml, which fails lint and all test jobs on every PR and on main. Happy to split it into its own PR if you prefer.

[cuinixam]

Nice catch 👍