feat(python): Support the most recent version of "duckdb_engine" connections via read_database

[alexander-beedie]

Closes #18263.

One of those rare two-character fixes... ;)

Following the "duckdb_engine" code a little more closely showed that we didn't need to expose the inner/private connection object to take advantage of the fetch_arrow_table fast path; the driver connection's __getattr__ accesses it implicitly for us. (Explicit access via .c was deprecated and removed recently, hence the reported error).

Confirmed the fix works and still takes the Arrow-aware fast path with old/new versions of the "duckdb_engine" alchemy driver.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.21%. Comparing base (c88da1f) to head (9b5e45a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/io/database/_executor.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18277      +/-   ##
==========================================
- Coverage   80.21%   80.21%   -0.01%     
==========================================
  Files        1500     1500              
  Lines      198897   198897              
  Branches     2837     2837              
==========================================
- Hits       159547   159540       -7     
- Misses      38822    38829       +7     
  Partials      528      528              

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