Pin unpinned JDBC behaviors before the refactor passes #195

Goal

Pin the four behaviors that the refactor passes touch but the suite does not currently check. A pin is a test that fails when the behavior changes. A test that cannot fail is not a pin.

This issue ships tests only. No production code changes.

Why

The suite is broad. ./mvnw -pl libsql-jdbc -am test runs 374 tests, and the xerial conformance suite adds 272. Four areas have no direct coverage, and three later passes move code in them.

Scope

• Attached-schema table metadata. Drive ATTACH then metadata lookups through LibsqlResultSetMetaData. Cover loadAttachedTableInfo and findSchema (LibsqlResultSetMetaData 468 to 501).
• Date and time tails. Cover a text value with an offset suffix, one with a Z suffix, a bare time value defaulting to 2000-01-01, and BLOB rejection with the "Bad value for type X" message (LibsqlResultSet 1156 to 1276).
• Savepoint edge cases. Cover two savepoints with the same name resolved by lastIndexOf, and releasing the outermost savepoint of a transaction started in auto-commit (LibsqlConnection 532 to 658).
• Concurrency edges. Cover batch execution racing close(). The socketTimeout URL case belongs to socketTimeout alone in a JDBC URL throws NumberFormatException #196 and is not part of this pass.

Mutation probe

For each new test, break the quirk in a scratch build and show the test fails. Revert the break. Record the command and the failure line in the PR body. A new test that stays green under the break is rewritten.

Acceptance

• The four areas above have characterization tests that fail against a deliberate perturbation.
• ./mvnw -pl libsql-jdbc -am test green, with no production file touched except a temporary reverted mutation.
• ./mvnw spotless:check green.
• The PR body lists each probe command and its observed failure.