Fix --mail-type args to use comma-separated values instead of repeated flags

[Copilot]

Description

On SLURM systems (e.g. Perlmutter), multiple mail types were being submitted as repeated flags (--mail-type end --mail-type fail) instead of a single comma-separated argument (--mail-type end,fail). This caused SLURM to silently drop the notification on job completion.

Root cause: The else branch in _submit_single_job was joining mail_type_args using the flag itself as separator, duplicating it:

# Before (broken): produces "--mail-type end --mail-type fail"
submitargs += " {} {}".format(
    mail_type_flag,
    " {} ".format(mail_type_flag).join(mail_type_args),
)

# After (fixed): produces "--mail-type end,fail"
submitargs += " {} {}".format(
    mail_type_flag,
    ",".join(mail_type_args),
)

The -m (PBS) path, which concatenates without any separator, is unchanged.

Fixes #4968

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation

[billsacks]

I don't feel a need to review this given others' reviews, so removing myself as reviewer.

[jasonb5]

LGTM