feat(python): Improve read_database interop with sqlalchemy Session and various Result objects

[alexander-beedie]

Closes #13926.

Further streamlines use of read_database by ensuring seamless interop with SQLAlchemy's Session and various Result objects (we were mostly fine, but some specific flavours of cursor result needed additional introspection to determine what columns were being returned).

• Improves column inference from cursor/result object (eg: ChunkedIteratorResult, as seen in the linked issue).
• Adds explicit test coverage for SQLAlchemy's Session object.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (815732b) 80.76% compared to head (01e323d) 80.78%.

Files	Patch %	Lines
py-polars/polars/io/database.py	88.88%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14557      +/-   ##
==========================================
+ Coverage   80.76%   80.78%   +0.01%     
==========================================
  Files        1326     1326              
  Lines      173035   173043       +8     
  Branches     2439     2443       +4     
==========================================
+ Hits       139758   139793      +35     
+ Misses      32805    32778      -27     
  Partials      472      472              

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