fix(runspec): reject unsupported executors in nemo-run pipeline launcher

[andrewwhitecdw]

Problem

PipelineConfig.executor is declared as Literal["local", "slurm", "dgxcloud", "lepton"], but run_nemo_run only special-cases "local":

if config.executor == "local":
    executor = run.LocalExecutor(...)
else:  # slurm
    ...
    executor = run.SlurmExecutor(...)

Every other value — including "dgxcloud" and "lepton" — falls into the else branch, so a user passing executor="dgxcloud" gets either a misleading --account required for Slurm executor error or, worse, a Slurm job silently submitted to the wrong backend.

Fix

Make the Slurm branch explicit (elif config.executor == "slurm") and reject anything else with a clear error:

[pipeline] ERROR: Executor 'dgxcloud' is not supported by the nemo-run pipeline launcher (supported: 'local', 'slurm')

Testing

• Verified dgxcloud/lepton now return rc=1 with the clear error; slurm without --account still hits its proper validation
• pytest tests/nemo_runspec — 186 passed, 3 pre-existing failures on main (unrelated data_mover/lepton tests, fail identically without this change)