fix: Set current_ptaskgroup only in async-with blocks

[achimnol]

• fix: Set current_ptaskgroup only in async-with blocks
• docs: Add a warning about the new behavior

[codecov]

Codecov Report

Merging #41 (50cd0eb) into main (f65cc97) will decrease coverage by 0.00%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   65.42%   65.41%   -0.01%     
==========================================
  Files          16       16              
  Lines        1284     1281       -3     
==========================================
- Hits          840      838       -2     
+ Misses        444      443       -1     

Impacted Files	Coverage Δ
src/aiotools/taskgroup/persistent.py	0.00% <0.00%> (ø)
src/aiotools/taskgroup/persistent_compat.py	83.24% <100.00%> (-0.19%)	⬇️

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