fix(python): Check for duplicate column names in read_database cursor result, raising DuplicateError if found

[alexander-beedie]

Closes #18526 👍

We now raise a proper DuplicateError if/when the database result cursor contains multiple instances of the same column name; currently we map the cursor description to a Schema dict without checking for this, which eats any duplicates - this then goes on to raise a much less obvious error later.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.80%. Comparing base (af8738c) to head (56df1ed).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/database/_executor.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18548      +/-   ##
==========================================
- Coverage   79.82%   79.80%   -0.03%     
==========================================
  Files        1502     1502              
  Lines      201933   201938       +5     
  Branches     2868     2869       +1     
==========================================
- Hits       161201   161161      -40     
- Misses      40186    40231      +45     
  Partials      546      546              

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