Publisher leaks a batch-actor task and unbounded channel per ordering key

Unconfirmed. This is the result of a search with Claude, may be a false positive.

In `src/pubsub/src/publisher/actor.rs`, publishing with an ordering key spawns a dedicated `BatchActor` task and adds an entry to `batch_actors`:

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/pubsub/src/publisher/actor.rs#L130-L147

```rust
// TODO(#4012): Remove batch actors when there are no outstanding operations on the ordering key.
```

Batch actors and their associated task handles and channels are never removed from `batch_actors`. In applications that use dynamic or short-lived ordering keys (e.g. per-entity or per-session keys), this creates an unbounded memory and background task leak.

Start in src/pubsub/src/publisher/actor.rs around the BatchActor creation at lines 130-147, then trace how batch_actors, task handles, and channels are managed for ordering keys. Verify the suspected leak with dynamic or short-lived keys. Done means actors and their associated resources are removed once an ordering key has no outstanding operations, without disrupting publishing.

Written by the indexing model from the issue text.