fix string representation of prescription and mar

[sainak]

Proposed Changes

• the string representation of mar and prescreptions was crashing the error message raised during cascade delete of consultaions

Merge Checklist

• Tests added/fixed
• Update docs in /docs
• Linting Complete
• Any other necessary step

Only PR's with test cases included and passing lint and test pipelines will be reviewed

@ohcnetwork/care-backend-maintainers @ohcnetwork/care-backend-admins

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 64.14%. Comparing base (c1a74f2) to head (6f3bf3d).
Report is 44 commits behind head on develop.

Files with missing lines	Patch %	Lines
care/facility/models/prescription.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2412      +/-   ##
===========================================
+ Coverage    63.82%   64.14%   +0.31%     
===========================================
  Files          239      241       +2     
  Lines        13416    13567     +151     
  Branches      1909     1941      +32     
===========================================
+ Hits          8563     8702     +139     
  Misses        4505     4505              
- Partials       348      360      +12     

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