Rollback open transactions after panics in client closures

[ryanfowler]

Summary

• Catch panics with access to the worker Connection so the runtime can recover the connection state before returning Error::Panic.
• Roll back any in-flight manual transaction when a user closure panics and the connection is not in autocommit mode.
• Add a regression test that panics after BEGIN IMMEDIATE and verifies the transaction is rolled back and the database is writable afterward.

Testing

• cargo fmt --all -- --check
• cargo test
• cargo test test_panic_after_begin_immediate_rolls_back
• cargo test test_closure_panic_surfaces_error
• cargo clippy -- -D warnings