Ensure rel_path is set when initializing FastAPI session

[philippjfr]

Fixes #7594

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.72%. Comparing base (dd534a5) to head (33b209e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7627      +/-   ##
==========================================
+ Coverage   84.99%   86.72%   +1.73%     
==========================================
  Files         341      341              
  Lines       51818    51819       +1     
==========================================
+ Hits        44042    44940     +898     
+ Misses       7776     6879     -897     

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