fix number of context planes

[braingram]

roman_datamodels defaults the context plane of the mosaic model to start with 2 planes:
https://github.com/spacetelescope/roman_datamodels/blob/5e996c37648bac7fa0ac3c4d5e0ce5b8ddaeb69a/src/roman_datamodels/maker_utils/_datamodels.py#L155

This is causing an extra context plane. This PR fixes the issue by setting n_images to 1.

Closes #1264

Regression tests show 2 expected failures (which shows the issue is fixed): https://github.com/spacetelescope/RegressionTests/actions/runs/10303608077

Checklist

• added entry in CHANGES.rst under the corresponding subsection
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.57%. Comparing base (155e8f3) to head (c5627f2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1355   +/-   ##
=======================================
  Coverage   78.57%   78.57%           
=======================================
  Files         117      117           
  Lines        7861     7861           
=======================================
  Hits         6177     6177           
  Misses       1684     1684           

Flag	Coverage Δ		*Carryforward flag
nightly	62.26% <ø> (ø)		Carriedforward from 155e8f3

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[schlafly]

Great, thanks Brett, approving.

[braingram]

Oooo, the CI picked up a new pars file:
https://roman-crds.stsci.edu/browse/roman_wfi_pars-jumpstep_0001.asdf
I guess that means pars files work :) and those tests are broken...

[schlafly]

Yeah, that's "good" I guess? I suspect this means some mixture of:

• we need to specify something like --disable-pars-files to avoid getting this file
• we need to specify some earlier context (?) that doesn't identify the new files
• we need to specify the rejection threshold in the tests (it looks to me like the primary change is that the detection threshold increases to 500 in the new files from 180 in the previous default, though I think my brief guess looking at the test would have been that the jump amplitude is ~70 sigma (500 / 7), so I wouldn't have originally expected it to be identified in either the new or old default)
• we need to disable the jump detection tests that we don't use.

We can discuss a little in 15 min at the tag up.