Fix logic when looking up pn.state.curdoc

[philippjfr]

In #5952 we refactored the try/finally block of pn.state.curdoc. However in doing so we didn't take into account that the finally block is always processed last and any value returned in the finally block takes precedence. We now refactor this block correctly to ensure that we always look up the curdoc in the ContextVar first ensuring that async callbacks return the correct curdoc.

Fixes #6252

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (2a892a3) 84.20% compared to head (4700b9d) 84.19%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/tests/ui/io/test_state.py	64.00%	9 Missing ⚠️
panel/io/state.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6254      +/-   ##
==========================================
- Coverage   84.20%   84.19%   -0.01%     
==========================================
  Files         301      301              
  Lines       45157    45190      +33     
==========================================
+ Hits        38023    38047      +24     
- Misses       7134     7143       +9     

Flag	Coverage Δ
ui-tests	40.74% <69.56%> (+0.02%)	⬆️
unitexamples-tests	71.83% <45.65%> (-0.03%)	⬇️

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