Await asyncs of components deleted before graceful shutdown

[rdlu]

Fixes #4400.

When components are deleted (delete_components/2 in channel.ex, i.e. on the client's cids_destroyed ack), their in-flight async pids are collected via the existing socket_asyncs/2 into a new orphaned_asyncs channel-state map and monitored; a dedicated handle_info :DOWN clause prunes entries (and keeps these monitor messages out of user handle_info/2). all_asyncs/1 merges the orphans, so graceful shutdown (await_asyncs/1) awaits them and LiveViewTest.render_async/2 waits for them too.

Notes for review:

• Deliberately no cancel-at-delete: killing the task at deletion just moves the mid-query tear-down earlier — Ensure async tasks can exit gracefully in LiveViewTest #4348's purpose is to let the work finish. Orphaned results are still discarded as today (write_component → :error → push_noop); only the shutdown/await bookkeeping changes.
• render_async/2 now also waits for asyncs of removed components (they share async_pids). We think that's a fix — previously a test could return while such a task was still writing to the DB — but it is an observable change; happy to narrow it to terminate/2 only if preferred.
• A separate Process.monitor/1 is needed for pruning because the monitor set up by Async.run_async_task/5 uses alias: :reply_demonitor and emits no :DOWN on successful completion.
• Regression tests live next to the existing "test supervisor shutdown awaits async tasks" test in test/phoenix_live_view/integrations/start_async_test.exs, using the [:phoenix, :live_component, :destroyed] telemetry event to deterministically wait for the actual delete_component (the client acks cids_destroyed asynchronously). Verified: new tests red on unpatched main, full suite green after the fix, and the single-file repro from the issue flips exactly as expected against the patched tree.

Disclosure: this fix was developed with AI assistance — Claude Fable 5 (analysis/orchestration) with Claude Opus 5 subagents (implementation), directed and reviewed by a human.