Preserve enclosing transactions with nested savepoints

[quinnj]

Calling DBInterface.transaction inside a transaction started with SQL currently sends another START TRANSACTION, which commits the caller's earlier work. A later caller rollback cannot undo those rows. Use the server transaction status after draining pending results to choose a savepoint for an existing transaction. Nested helper calls and prepared executemany then compose without committing their enclosing scope.

Keep the connection locked and restore its previous task owner. If rollback or savepoint cleanup fails, close the uncertain session and report both the original and cleanup errors. Reuse the existing connection token sequence for reserved _mysqljl_ savepoint names; no new dependency, export or connection field.

MySQL statements that implicitly commit, including table creation in MySQL.load, and explicit transaction controls inside the callback remain outside this guarantee. The docs state those boundaries.

Validation:

• Real MySQL 8.4.11 reproduces three failures on pristine main on both Julia 1.10.12 and 1.13.0; the same six controls pass with the fix. The permanent public workflow covers manual/implicit outer transactions, three-level nesting, caught inner errors, prepared-batch rollback and caller rollback.

• Full minimum-version suite: 2,267 assertions, including MySQL 8.4 and MariaDB 11.4 live workflows and lifecycle soak. Current protocol suite: 1,900 assertions. Heavy unrelated scan-performance gates were disabled. Strict documentation and independent Astra source review pass.

• The actual safe-trim executable passes seven runtime controls with code generation disabled, including nested transactions and failed rollback cleanup. Runtime-schema column materialization remains outside that fixture's supported subset.

• Eight matched warm-cost processes retain exactly the same allocations for ordinary top-level transactions on both runtimes: 704/832 bytes for empty/ping callbacks on current, 864/1,024 on minimum. Local timing remains within the paired variation. Separate first-use controls show a 5–13 ms increase per new callback shape, primarily compilation; this is a correctness/composition change, not a speedup claim.

• Hosted CI at 49acd08: all 15 applicable checks green, including Docker integration and all platform/version lanes; 2 scheduled-only gates skipped.

Co-authored by Codex