refactor(rails): introduce worker_thread harness hook for the hub-isolation example

The worker_hub_isolation shared example previously hard-coded
Thread.new for the two concurrent jobs it spawns. That works fine on
adapters that keep their queue state in-process (:test, :inline) but
some real adapters need per-thread setup (e.g. :solid_queue on SQLite
needs an isolated database per worker thread to avoid
SQLite3::BusyException).

Adds a `worker_thread(&block)` hook on the harness, defaulting to
`Thread.new(&block)`, and switches the shared example to call it.
Adapters that need extra worker setup override the hook.

Behaviour on :test (the only adapter on this branch) is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: solnic

sentry-rails/spec/active_job/shared_examples/tracing/worker_hub_isolation.rb

@@ -20,8 +20,8 @@ def perform
 
     Sentry.get_current_scope.set_tags(test_thread: true)
 
-    thread_a = Thread.new { job_a.perform_later; drain }
-    thread_b = Thread.new { job_b.perform_later; drain }
+    thread_a = worker_thread { job_a.perform_later; drain }
+    thread_b = worker_thread { job_b.perform_later; drain }
     [thread_a, thread_b].each(&:join)
 
     txn_a = transactions.find { |t| t.tags[:job] == "A" }

sentry-rails/spec/active_job/support/harness.rb

@@ -69,4 +69,12 @@ def within_parent_transaction(name: "parent.test", op: "test")
   ensure
     txn&.finish
   end
+
+  # Hook used by the worker_hub_isolation shared example. The default
+  # is a plain Thread.new — adapters that need extra setup (e.g. an
+  # isolated database per worker thread, like :solid_queue on SQLite)
+  # override this to wrap the block in their isolation scope.
+  def worker_thread(&block)
+    Thread.new(&block)
+  end
 end