test: verify Live grouper disposal on a shared Python WebSocket (#3977)

When two transcript groupers observe the same Live dispatcher, callers
must be able to detach and close one without stopping the other or
losing the event stream. The existing grouper tests covered its own
listeners and timers but did not exercise an attached connection.

This tests both sync and async clients over a real local WebSocket. It
checks that the first grouper cancels its active timer, typed and
wildcard observers keep receiving the original values and future
metadata, the second grouper continues accumulating, and the same socket
accepts a post-disposal update. The peer waits for explicit caller
teardown. Tests only; no generated or API changes.

Validation:
- 238 Live tests passed on Pydantic 2.12.5 and 238 on 1.10.26.
- After typing the close callbacks and checking only event IDs present
on the wire: 14 socket-role and grouper tests pass; Pyright reports 0
errors on the affected file.
- Repository-configured `mypy .` passes across 1,920 source files; Ruff
check, format and diff checks pass.