test(image): bind the fixture Flask server to an ephemeral port

The URL-image tests hard-coded port 5555 and never shut the server down,
so a leftover or concurrent run left the port occupied and the readiness
loop fell through silently — the tests then ran against whatever was
listening, or against nothing.

Use werkzeug's make_server on port 0, record the assigned port, and shut
the server down in tearDownClass. The readiness loop now raises instead
of falling through when the server never comes up.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/abstract/test_abstract_blocks.py

@@ -307,6 +307,8 @@ class TestImagePIL(unittest.TestCase):
 
         if FLASK_AVAILABLE and hasattr(cls, 'flask_thread'):
             cls.flask_server_running = False
+            cls.flask_server.shutdown()
+            cls.flask_server.server_close()
             cls.flask_thread.join(timeout=2)
 
     @classmethod
@@ -350,10 +352,9 @@ class TestImagePIL(unittest.TestCase):
         """Start a Flask server for URL testing."""
         import urllib.request
         import urllib.error
+        from werkzeug.serving import make_server
 
         cls.flask_app = Flask(__name__)
-        cls.flask_port = 5555  # Use a specific port for testing
-        cls.flask_server_running = True
 
         @cls.flask_app.route('/test.jpg')
         def serve_test_image():
@@ -363,11 +364,12 @@ class TestImagePIL(unittest.TestCase):
         def health_check():
             return 'OK', 200
 
-        def run_flask():
-            cls.flask_app.run(host='127.0.0.1', port=cls.flask_port, debug=False,
-                              use_reloader=False, threaded=True)
+        # Bind to an ephemeral port so concurrent/leftover test runs can't clash
+        cls.flask_server = make_server('127.0.0.1', 0, cls.flask_app, threaded=True)
+        cls.flask_port = cls.flask_server.server_port
+        cls.flask_server_running = True
 
-        cls.flask_thread = threading.Thread(target=run_flask, daemon=True)
+        cls.flask_thread = threading.Thread(target=cls.flask_server.serve_forever, daemon=True)
         cls.flask_thread.start()
 
         # Wait for server to be ready with health check
@@ -379,12 +381,15 @@ class TestImagePIL(unittest.TestCase):
             try:
                 with urllib.request.urlopen(f'http://127.0.0.1:{cls.flask_port}/health', timeout=1) as response:
                     if response.status == 200:
-                        break
+                        return
             except (urllib.error.URLError, ConnectionRefusedError, OSError):
                 pass
             time.sleep(wait_interval)
             elapsed += wait_interval
 
+        raise RuntimeError(
+            f"Test Flask server did not become ready on port {cls.flask_port} within {max_wait}s")
+
     def test_image_url_detection(self):
         """Test URL detection functionality."""
         img = Image()