Coupler hardening and correctness revisions

[jsauer-NCAR]

Expected Differences

• Do these changes introduce new tools, command line arguments, or configuration file options? [Yes or No]
  
  If yes, please describe:
  
  No

Pull Request Testing

• Describe testing already performed for these changes:
  
  Tested on several cases which previous coupler showed unexpected shift in nested FE domain and/or inappropriate extrapolation at upper-bound in the x and/or y directions.
• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  N/A
• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]
  N/A
• Do these changes include sufficient testing updates? [Yes or No]
  N/A
• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  
  No
• Please complete this pull request review by [Fill in date].
  
  9/12/25

Pull Request Checklist

• Complete the PR definition above.
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