fix(auth): allow listeners to unsubscribe during notification

[hsusul]

Fixes #1642.

An auth listener calling its own unsubscribe() during sign_out() mutates the live subscriber dictionary, raising RuntimeError and preventing later listeners from receiving the event. Snapshot the subscribers before dispatch in both clients so the current notification completes and the removed listener stays absent from subsequent events.

Adds public-API regression tests for sync and async clients; sync files were generated with the package's unasync script.

Validation (Python 3.13.12):

• Both regression tests failed with the dictionary-mutation error on upstream bb7ecc5, then passed with the fix.
• Full auth suite: 166 passed, with repository Docker Compose services. Used an isolated Compose project and omitted the database's host-port mapping to avoid an existing local port conflict.
• Package mypy, Ruff lint/format checks, and git diff --check passed.

cc @olirice @silentworks