test: route unit-test threads through the platform thread helper

Spawn the threads in these concurrency tests via
google.adk.platform.thread.create_thread instead of constructing
threading.Thread directly. Behavior is unchanged: create_thread returns a
real OS thread, and the tests still use threading.Event/Barrier for
coordination.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 951583146

Author: GWeale

tests/unittests/memory/test_in_memory_memory_service.py

@@ -17,6 +17,7 @@
 
 from google.adk.events.event import Event
 from google.adk.memory.in_memory_memory_service import InMemoryMemoryService
+from google.adk.platform import thread as platform_thread
 from google.adk.sessions.session import Session
 from google.genai import types
 import pytest
@@ -438,9 +439,9 @@ def reader():
       loop.close()
 
   threads = [
-      threading.Thread(target=writer),
-      threading.Thread(target=reader),
-      threading.Thread(target=reader),
+      platform_thread.create_thread(writer),
+      platform_thread.create_thread(reader),
+      platform_thread.create_thread(reader),
   ]
   for thread in threads:
     thread.start()

tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py

@@ -31,6 +31,7 @@
 from google.adk.events import event_actions as event_actions_lib
 from google.adk.models import llm_request as llm_request_lib
 from google.adk.models import llm_response as llm_response_lib
+from google.adk.platform import thread as platform_thread
 from google.adk.plugins import bigquery_agent_analytics_plugin
 from google.adk.plugins import plugin_manager as plugin_manager_lib
 from google.adk.sessions import base_session_service as base_session_service_lib
@@ -10357,7 +10358,9 @@ def run_in_fresh_loop():
       except BaseException as e:  # noqa: BLE001 - collecting for assertion
         errors.append(e)
 
-    threads = [threading.Thread(target=run_in_fresh_loop) for _ in range(2)]
+    threads = [
+        platform_thread.create_thread(run_in_fresh_loop) for _ in range(2)
+    ]
     for t in threads:
       t.start()
     for t in threads:
@@ -10487,7 +10490,9 @@ def run_in_fresh_loop():
       except BaseException as e:  # noqa: BLE001
         errors.append(e)
 
-    threads = [threading.Thread(target=run_in_fresh_loop) for _ in range(2)]
+    threads = [
+        platform_thread.create_thread(run_in_fresh_loop) for _ in range(2)
+    ]
     for t in threads:
       t.start()
     # Deterministic rendezvous: hold the owner inside setup until BOTH
@@ -10534,7 +10539,9 @@ def run_in_fresh_loop():
       except BaseException as e:  # noqa: BLE001
         errors.append(e)
 
-    threads = [threading.Thread(target=run_in_fresh_loop) for _ in range(2)]
+    threads = [
+        platform_thread.create_thread(run_in_fresh_loop) for _ in range(2)
+    ]
     for t in threads:
       t.start()
     entered.wait(timeout=5)