Improvements to prompt building

[edgararuiz]

• Cleans up and prompts
• Changes default of prompt output from list to a character vector
• In R, resorts the methods to make sure the stack properly
• Updates tests

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.74%. Comparing base (ddb1f76) to head (d215bdd).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
r/R/m-backend-prompt.R	90.00%	7 Missing ⚠️
python/mall/llm.py	72.22%	5 Missing ⚠️
python/mall/prompt.py	96.15%	1 Missing ⚠️
r/R/m-backend-submit.R	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
- Coverage   98.80%   97.74%   -1.07%     
==========================================
  Files          29       29              
  Lines        1092     1064      -28     
==========================================
- Hits         1079     1040      -39     
- Misses         13       24      +11     

Flag	Coverage Δ
unittests	96.90% <86.95%> (-0.77%)	⬇️

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