bug(stream): removing a listener during join notification skips a sibling callback #2029

Describe the bug

In SDK v12.12.0, removing a listener inside its own subscription callback can cause the next caller on the same topic to miss the join result. This is a follow-up edge case to the callback cleanup added for #2027, not the original reconnect bug.

Steps to reproduce

Using the existing mock WebSocket and PhoenixChannelsTransport helpers (no OpenSea API calls):

1. Subscribe callers A and B to collection:c1. Each registers an ITEM_SOLD handler and an onSubscribed callback.
2. In A's onSubscribed, call the unsubscribe function returned by A's .on(...).
3. Acknowledge the initial phx_join.

A focused test on current main fails at expect(second).toHaveBeenCalledTimes(1) with 0 calls. The same behavior occurs if A removes its handler inside onSubscribeError after a rejected join.

Expected behavior

A and B should each receive one join-result notification. Removing A should not skip B.

Environment

• Package version: 12.12.0 (main at 8ed5cd6)
• Node.js version: 22.21.0
• Operating system: macOS 15.6.1
• Chain / network: none; offline mock WebSocket

Additional context

callerHandle() splices A from subscription.callbackList when its last handler is removed, while sendJoin() iterates that same array. B shifts into A's index and is skipped. The success, error and timeout paths all iterate the live list.

Locally, iterating a snapshot in those three paths makes both new regression tests pass. With that small change, all 54 transport tests and 1,302 SDK unit tests pass, as do check-types, build, and Biome on the affected files. No live stream or API request was made.

Since this repository mirrors an internal monorepo, I am filing the reproducible bug here first rather than assuming a PR can be merged directly. I can share the focused test and patch if useful.