chore: make default value of enable_tracing in ADK preview template to match the GA one

This will allow for default-on telemetry with preview template if enable_tracing was not specified.

See https://github.com/googleapis/python-aiplatform/blob/9a46e67a8c341673b14bece88bc635b455314711/vertexai/agent_engines/templates/adk.py#L481.

PiperOrigin-RevId: 827514536

Author: vertex-sdk-bot

vertexai/preview/reasoning_engines/templates/adk.py

@@ -491,7 +491,7 @@ def __init__(
         *,
         agent: "BaseAgent",
         plugins: Optional[List["BasePlugin"]] = None,
-        enable_tracing: bool = False,
+        enable_tracing: Optional[bool] = None,
         session_service_builder: Optional[Callable[..., "BaseSessionService"]] = None,
         artifact_service_builder: Optional[Callable[..., "BaseArtifactService"]] = None,
         memory_service_builder: Optional[Callable[..., "BaseMemoryService"]] = None,