Support Python 3.11 and earlier in authentication file error handling

[mmguero]

Description

The authentication file validation code introduced in 26.07.0 for #865 (see mmguero-dev@954daf7) introduced a multiline expression inside a single-quoted f-string:

missingAuthMessage = f'Files relating to authentication and/or secrets are missing: {", ".join([
    p[len(malcolmPathPrefix) :] if p.startswith(malcolmPathPrefix) else p for p in missingAuthFiles
])}; please run ./scripts/auth_setup to generate them'

This syntax works with Python 3.12 and later due to the updated f-string grammar introduced by PEP 701.

On Python 3.11 and earlier, however, the script fails to parse with:

SyntaxError: unterminated string literal

This prevents commands such as start, wipe, and logs from running on systems using an older supported Python version.

Fix

Build the comma-separated file list separately, then interpolate the resulting string into the error message:

missingAuthFileList = ", ".join(
    p[len(malcolmPathPrefix) :]
    if p.startswith(malcolmPathPrefix)
    else p
    for p in missingAuthFiles
)

missingAuthMessage = (
    "Files relating to authentication and/or secrets are missing: "
    f"{missingAuthFileList}; please run ./scripts/auth_setup to generate them"
)

This preserves the existing behavior while remaining compatible with Python 3.11 and newer.

See idaholab@b1d0700 for the fix.