Close internally prepared statements when execution fails

[quinnj]

The generic execute(connection, sql, params) and executemultiple(connection, sql, params) helpers prepare a statement internally. If execution then throws before returning a cursor, that statement was left open.

Close the internally prepared statement on that failure path and rethrow. Successful results retain their existing lifetime, and explicitly supplied statements remain caller-owned. Document this ownership rule. Driver-specific connection overrides and failures while consuming a returned cursor remain outside these generic helpers.

Validation: two new cleanup assertions failed before the fix. All ten ownership assertions now pass, including successful-result and caller-owned-statement controls. Full package suites pass on Julia 1.9.4 and 1.12.6 with four threads. The regression uses the existing fake driver; no production leak measurement is claimed.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.92%. Comparing base (53901ea) to head (9f6643d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   82.40%   85.92%   +3.52%     
==========================================
  Files           1        1              
  Lines         125      135      +10     
==========================================
+ Hits          103      116      +13     
+ Misses         22       19       -3     

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