Fix poller thread cleanup for test stability

Move thread join into reset (test-only path) so stop stays
fire-and-forget in production. Suppress report_on_exception to
avoid spurious stderr output when threads are killed mid-request.

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

Author: jnunemaker

lib/flipper/poller.rb

@@ -18,7 +18,10 @@ def self.get(key, options = {})
     end
 
     def self.reset
-      instances.each {|_, instance| instance.stop }.clear
+      instances.each do |_, instance|
+        instance.stop
+        instance.thread&.join(1)
+      end.clear
     end
 
     MINIMUM_POLL_INTERVAL = 10
@@ -116,6 +119,7 @@ def ensure_worker_running
       begin
         return if thread_alive?
         @thread = Thread.new { run }
+        @thread&.report_on_exception = false
         @instrumenter.instrument("poller.#{InstrumentationNamespace}", {
           operation: :thread_start,
         })