Release pending transactions when NativeTransaction is garbage-collected

[kriszyp]

Parent: #741

Problem

Transaction::Constructor registers every native handle in DBDescriptor::transactions with a strong shared_ptr. The NativeTransaction finalizer only resets the JS-side pointer. If an uncommitted transaction becomes unreachable while its database and environment remain alive, the descriptor registry keeps the handle alive, so its RocksDB transaction, snapshot, DB handle, VT intents, and transaction-log pending state can remain retained until environment or database shutdown.

PR #780 reaps these handles when their environment exits. That bounds the worker-recycle failure but does not give an abandoned transaction the lifetime of its JS owner in a long-lived environment.

Required invariant

An unreachable, uncommitted NativeTransaction must not retain native transaction resources indefinitely. Cleanup must unregister and settle the handle without racing in-flight native work.

Acceptance criteria

• Close/abort safely from the JS finalizer, or change registry ownership so the registry cannot be the last strong owner of an otherwise unreachable transaction.
• Reuse the definitive close/work synchronization established for fix: close pending transactions with their owning DBHandle; napi-free TransactionHandle::close (#741) #780/Make TransactionHandle close exclude new work and fully drain in-flight operations #784; do not introduce a finalizer-versus-commit race.
• Preserve explicit commit, abort, retry, outstanding-iterator, and transaction-log semantics.
• Add a forced-GC regression against a long-lived open database.
• Verify that the descriptor transaction count returns to baseline and that snapshots, VT intents, and transaction-log pending state are released.

Relationship

• Umbrella: Worker-env teardown destroys transactions on the shared DBDescriptor, corrupting the heap under concurrent commits #741
• Environment-exit reap: fix: close pending transactions with their owning DBHandle; napi-free TransactionHandle::close (#741) #780
• Close synchronization prerequisite: Make TransactionHandle close exclude new work and fully drain in-flight operations #784

— KrAIs (Codex), on behalf of @kriszyp