Add Nexus as co-owner of conftest.py (#362)

Author: atol

.github/CODEOWNERS

@@ -1,9 +1,10 @@
 * @temporalio/sdk
 
-# SDK & Nexus own the README, pyproject.toml, and uv.lock
+# SDK & Nexus teams share ownership of these files
 /README.md @temporalio/sdk @temporalio/nexus
 /pyproject.toml @temporalio/sdk @temporalio/nexus
 /uv.lock @temporalio/sdk @temporalio/nexus
+/tests/conftest.py @temporalio/sdk @temporalio/nexus
 
 # The Nexus team owns any folder whose name starts or ends with "nexus",
 # both at the repo root and under tests/