Clean up dead thread from process_instances when replacing it

When a thread dies, `replace_thread` deleted its entry from
`configured_processes` but not from `process_instances`. On the next
supervise loop, `check_and_replace_terminated_processes` would find the
same dead thread again, call `replace_thread`, and
`configured_processes.delete(thread_id)` would return nil — crashing
with `undefined method 'instantiate' for nil`.

Mirror the pattern already used in `ForkSupervisor#replace_fork`:
delete from `process_instances` first and guard against the entry
already being gone.

Fixes #710

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rosa

lib/solid_queue/async_supervisor.rb

@@ -19,17 +19,19 @@ def supervise
 
       def check_and_replace_terminated_processes
         terminated_threads = process_instances.select { |thread_id, instance| !instance.alive? }
-        terminated_threads.each { |thread_id, instance| replace_thread(thread_id, instance) }
+        terminated_threads.each { |thread_id, _| replace_thread(thread_id) }
       end
 
-      def replace_thread(thread_id, instance)
+      def replace_thread(thread_id)
         SolidQueue.instrument(:replace_thread, supervisor_pid: ::Process.pid) do |payload|
-          payload[:thread] = instance
+          if (instance = process_instances.delete(thread_id))
+            payload[:thread] = instance
 
-          error = Processes::ThreadTerminatedError.new(instance.name)
-          release_claimed_jobs_by(instance, with_error: error)
+            error = Processes::ThreadTerminatedError.new(instance.name)
+            release_claimed_jobs_by(instance, with_error: error)
 
-          start_process(configured_processes.delete(thread_id))
+            start_process(configured_processes.delete(thread_id))
+          end
         end
       end