Rag Tools fix + relaxing const for API key loading

[svilupp]

• Fixed loading RAGResult when one of the candidate fields was nothing.
• Utility type checks like isusermessage, issystemmessage, isdatamessage, isaimessage, istracermessage do not throw errors when given any arbitrary input types (previously they only worked for AbstractMessage types). It's a isa check, so it should work for all input types.
• Changed preference loading to use typed global instead of const, to fix issues with API keys not being loaded properly on start.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.19%. Comparing base (da4089f) to head (de5c780).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
- Coverage   92.37%   92.19%   -0.19%     
==========================================
  Files          44       44              
  Lines        3622     3653      +31     
==========================================
+ Hits         3346     3368      +22     
- Misses        276      285       +9     

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