Fix a bug in _get_function_body_without_inners for module sdks.python.transforms.core

[DKER2]

Fixes #28061
Some functions are defined in a class like this:

class A:
     @Tagging
     def process():

The current function does not remove @tagging because the line begins with a space, rather than directly with the @ symbol. This will lead to miss-warning of class apache_beam.dataframe.convert.DataFrameToRowsFn

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.32%. Comparing base (160dffd) to head (f07d2cd).
Report is 11 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #32590      +/-   ##
============================================
- Coverage     57.32%   57.32%   -0.01%     
  Complexity     1474     1474              
============================================
  Files           967      967              
  Lines        153754   153758       +4     
  Branches       1076     1076              
============================================
- Hits          88140    88139       -1     
- Misses        63410    63415       +5     
  Partials       2204     2204              

Flag	Coverage Δ
python	81.40% <100.00%> (-0.01%)	⬇️

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