Support keyword batches on prepared statements

[quinnj]

executemany(conn, sql; id=[1, 2]) works, but the same keyword batch against an already prepared statement raises MethodError. The documented no-parameter executemany(stmt) form also has no method.

Add the prepared-statement keyword wrapper, delegating to the existing batch implementation like the connection form. Keyword batches retain positional field-order semantics. Tests cover batch values, invalid lengths, empty batches, no parameters, cursor cleanup, and keeping the caller's statement open.

Validation: the missing-method reproduction fails on master. Full test suites pass on Julia 1.9.4 and 1.12.6 with four threads.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.40%. Comparing base (95da06b) to head (80a3fcc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   82.25%   82.40%   +0.14%     
==========================================
  Files           1        1              
  Lines         124      125       +1     
==========================================
+ Hits          102      103       +1     
  Misses         22       22              

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