fix: Handle missing registry in Subscription.unsubscribe/2 during shutdown

[perzanko]

Problem

When using SSE-based subscriptions (standard_sse: true in absinthe_plug), active connections call unsubscribe/2 during connection teardown. In a standard supervision tree where Absinthe.Subscription is started after the Endpoint (as documented), OTP terminates children in reverse start order — meaning the subscription Registry is shut down before the Endpoint finishes draining connections.

This causes an ArgumentError: unknown registry from Registry.unregister_match/4 during every deployment for each active SSE subscription.

Elixir.ArgumentError: unknown registry: MyAppWeb.Endpoint.Registry
    (elixir 1.19.5) lib/registry.ex:1551: Registry.info!/1
    (elixir 1.19.5) lib/registry.ex:1073: Registry.unregister_match/4
    (absinthe 1.9.0) lib/absinthe/subscription.ex:183: anonymous fn/4 in Absinthe.Subscription.unsubscribe/2
    ...
    (absinthe_plug 1.5.9) lib/absinthe/plug.ex:399: Absinthe.Plug.subscribe_loop/3

Fix

Guard against the missing registry using Process.whereis/1 in unsubscribe/2, following the same pattern already used in extract_pubsub/2.

The process dictionary cleanup (pdict_delete_fields) always runs regardless, so there's no resource leak.