docs(tools): encourage using mode='single_turn' in AgentTool and document manual artifacts configuration

Merge #6197

### Link to Issue or Description of Change
- Closes: #3232

### Problem
PR 6197 originally proposed adding an auto-attachment flag to `AgentTool` to recursively inject `load_artifacts_tool` to sub-agents. However:
1. `AgentTool` is no longer encouraged to use directly in favor of using `mode='single_turn'` sub-agents.
2. Auto-attaching the tool dynamically at runtime via `list_artifacts` creates unnecessary performance overhead for all agent tool executions.

### Solution
Instead of any code changes, we update `AgentTool`'s docstring to:
1. Recommend the use of `mode='single_turn'` sub-agents (the modern replacement for `AgentTool`).
2. Document that if a sub-agent needs to access parent artifacts, the developer should simply add `load_artifacts_tool` manually to the sub-agent's `tools` list.

This resolves the usability gap by clarifying the path forward without adding deprecated flags or introducing runtime slowness.

Co-authored-by: Xuan Yang <xygoogle@google.com>
COPYBARA_INTEGRATE_REVIEW=#6197 from nuthalapativarun:fix/3232-auto-include-load-artifacts-tool 645c99e
PiperOrigin-RevId: 948575713

Author: 2 people

src/google/adk/tools/agent_tool.py

@@ -117,7 +117,13 @@ class AgentTool(BaseTool):
     setting ``mode='single_turn'`` on the sub-agent and attaching it via
     ``sub_agents=[...]`` instead of wrapping it with ``AgentTool``. The
     framework then exposes the sub-agent as a tool automatically and runs it
-    inline in the parent's session. See the single-turn mode guide for details.
+    inline in the parent's session.
+
+    If the sub-agent needs to access parent artifacts, add
+    ``load_artifacts_tool`` directly to the sub-agent's ``tools`` list.
+
+    Direct usage of ``AgentTool`` is discouraged. See the single-turn
+    mode guide for details.
 
   Attributes:
     agent: The agent to wrap.