Improve code quality base on DeepSource

[tatiana]

During the development of PR #296, DeepSource identified:

• Method '_table_exists_in_db' is abstract in class 'SqlDecoratedOperator' but is not overridden
• Method 'handle_dataframe_func' is abstract in class 'SqlDecoratedOperator' but is not overridden

In several subclasses, as part of:

• https://deepsource.io/gh/astro-projects/astro/run/99bc601f-513a-4dbc-b771-65cf795aa2aa/python/PYL-W0223

[codecov]

Codecov Report

Merging #299 (c473be6) into main (f9a50f4) will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
+ Coverage   92.05%   92.27%   +0.22%     
==========================================
  Files          35       35              
  Lines        1535     1527       -8     
  Branches      268      268              
==========================================
- Hits         1413     1409       -4     
+ Misses         90       86       -4     
  Partials       32       32              

Impacted Files	Coverage Δ
src/astro/sql/operators/sql_decorator.py	92.14% <ø> (+0.86%)	⬆️
src/astro/sql/parsers/sql_directory_parser.py	94.39% <ø> (+1.59%)	⬆️

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