Differential Fuzzing (3/3): Check DuckDbDialect against DuckDB's own parser

[LucaCappelletti94]

This PR adds fuzz_duckdb_accepts, a target that parses each input with a bundled DuckDB through json_serialize_sql and fails when DuckDB accepts a SELECT that DuckDbDialect rejects. Nothing is executed or bound. json_serialize_sql only serializes SELECT statements, so other statements are skipped rather than risk false reports. DuckDB's grammar also turns any unknown operator string, such as the trailing & in SELECT 1 &, into a call to a function that does not exist, so inputs calling an operator missing from DuckDB's function catalog are skipped too.

Inputs the dialect accepts more liberally than DuckDB never fail.

A 5-minute run found several distinct gaps.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.96%. Comparing base (4a609e7) to head (ea3788b).

Additional details and impacted files

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##             main    #2564   +/-   ##
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  Coverage   80.96%   80.96%           
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  Files          42       42           
  Lines       33359    33359           
  Branches    33359    33359           
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  Hits        27009    27009           
  Misses       2790     2790           
  Partials     3560     3560           

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