fix(python): Fix DataFrame.write_database(..., if_table_exists="append", engine="adbc") not handling missing tables correctly

[ilya-pevzner]

Why these changes?

This PR addresses issue #26906 where df.write_database(..., if_table_exists="append", engine="adbc") failed if the target table did not exist. The expected behavior for "append" is often to create the table if missing.

What was changed?

• The mode parameter for ADBC write_database is now dynamically set to "create_append" when adbc-driver-manager version is >= 0.7.0.
• For older adbc-driver-manager versions (< 0.7.0), the mode remains "append" to maintain backward compatibility, as "create_append" was introduced in ADBC 0.7.0.
• A new unit test, test_write_database_append_creates_missing_table, has been added to verify that if_table_exists="append" correctly creates a missing table for both SQLAlchemy and ADBC engines. The ADBC part of the test is skipped if the adbc-driver-manager version is too old to support "create_append".

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1. I used AI to implement the version-gated logic for ADBC mode and to add the corresponding unit test.
2. I confirm that I have reviewed all changes myself, and I believe they are relevant and correct.

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As requested, since I am a first time contributor, I am including a screenshot of me successfully running the test suite (make test) locally on my machine (not the CI).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.74%. Comparing base (3ee3f1b) to head (c79db83).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26913   +/-   ##
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  Coverage   81.73%   81.74%           
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  Files        1806     1806           
  Lines      248701   248701           
  Branches     3133     3133           
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+ Hits       203287   203290    +3     
+ Misses      44609    44606    -3     
  Partials      805      805           

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