atm nx and ny attributes are incorrect for single-column runs

### What happened?

When running a single-column case, the atm nx and ny attributes passed to CMEPS are incorrectly set to global sizes rather than 1.

### What are the steps to reproduce the bug?

This is observable in case2 of SCT - e.g., `SCT_D_Ln7.ne3_ne3_mg37.QPC6.derecho_intel.cam-scm_prep_c6` or `SCT_D_Ln7.ne3_ne3_mg37.QPC5.derecho_intel.cam-scm_prep`. Observe the dimension sizes of the cpl hist file: the nx is 488 for atm dimensions, when this should be 1. (The first grid cell is filled with the actual data.)

### What CAM tag were you using?

cam6_4_173

### What machine were you running CAM on?

CISL machine (e.g. derecho)

### What compiler were you using?

Intel

### Path to a case directory, if applicable

/glade/derecho/scratch/sacks/cesm3_0_alpha09d_fixscol_0715a/SCT_D_Ln7.ne3_ne3_mg37.QPC6.derecho_intel.cam-scm_prep_c6.C.20260716_092504_k4ahu7/run/case2run

### Will you be addressing this bug yourself?

Yes

### Extra info

As far as I know, this doesn't cause any significant problems. But it leads to larger-than-necessary cpl hist and cpl restart files, and could potentially lead to future issues.

I have a suggested fix incoming.