Fix type annotations on convert_app(s)

[philippjfr]

Fixes #3945

[codecov]

Codecov Report

Merging #4342 (5f1fd46) into main (56a64cb) will decrease coverage by 0.16%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #4342      +/-   ##
==========================================
- Coverage   82.77%   82.61%   -0.16%     
==========================================
  Files         240      240              
  Lines       34599    34603       +4     
==========================================
- Hits        28639    28587      -52     
- Misses       5960     6016      +56     

Flag	Coverage Δ
ui-tests	37.99% <50.00%> (-0.30%)	⬇️
unitexamples-tests	73.80% <0.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
panel/io/convert.py	71.77% <50.00%> (-0.36%)	⬇️
panel/tests/ui/widgets/test_misc.py	33.33% <0.00%> (-64.11%)	⬇️
panel/tests/ui/io/test_jupyter_server_extension.py	50.00% <0.00%> (-50.00%)	⬇️
panel/tests/ui/io/test_jupyterlite.py	89.74% <0.00%> (-7.70%)	⬇️
panel/tests/util.py	87.37% <0.00%> (-3.89%)	⬇️
panel/tests/ui/io/test_convert.py	93.10% <0.00%> (-2.59%)	⬇️
panel/tests/conftest.py	88.08% <0.00%> (-1.28%)	⬇️
panel/io/state.py	70.46% <0.00%> (-0.18%)	⬇️
panel/widgets/indicators.py	72.24% <0.00%> (-0.16%)	⬇️

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