fix(test): confirm the seed segment is registered before the concurrent pair

The reporter thread delivers with up to ~20ms poll latency while the test
body fires /users milliseconds after prepare, so on slow runners the
/ping seed could still be in flight together with both /users reports and
lose the collector's first-insert race itself. Poll /receiveData (read-
only) until the seed is registered before triggering the concurrent
reports.

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

Author: wu-sheng

tests/plugin/web/sw_fork_support/test_fork_support.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+import time
 from typing import Callable
 
 import pytest
@@ -43,6 +44,16 @@ class TestPlugin(TestPluginBase):
 
     @pytest.mark.parametrize('version', ['grpcio>=1.83'])
     def test_plugin(self, docker_compose, version):
+        # the /ping seed segment must be REGISTERED by the collector before /users makes
+        # the parent and child report concurrently, otherwise all three segments can be
+        # in flight together and the collector's first-insert race still drops one
+        for _ in range(30):
+            if '/ping' in requests.get('http://localhost:12800/receiveData', timeout=5).text:
+                break
+            time.sleep(1)
+        else:
+            raise Exception('the /ping seed segment never reached the collector')
+
         response = requests.get('http://0.0.0.0:9090/users', timeout=5)
         assert response.status_code == 200