Fix DBInterface transaction values and NULL binding

[quinnj]

Summary

• return the transaction closure value after a successful commit
• accept nothing as SQL NULL, matching the existing missing behavior
• add MySQL integration coverage for both cases

Root cause

DBInterface.transaction discarded the closure result and returned the C API commit result. The prepared-statement binder had a null path only for Missing, so Nothing fell through to a MethodError.

Scope

This addresses behaviors 2 and 3 from #238. The connection-level DBInterface.execute(conn, sql, params) form remains unsupported. Implementing it safely requires a statement-lifetime design because query cursors can outlive the call.

Validation

• full test suite passed against mysql:8: 200 tests
• verified a prepared statement can bind nothing, a concrete value, and missing in sequence
• verified a committed transaction returns the closure value
• git diff --check

Refs #238

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.89%. Comparing base (9db9900) to head (1c727f2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   69.82%   69.89%   +0.07%     
==========================================
  Files          10       10              
  Lines        1183     1186       +3     
==========================================
+ Hits          826      829       +3     
  Misses        357      357              

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