fix: remove developer notes from transfer_to_agent docstring

Merge #4617

## Summary

Fixes #4615

The `transfer_to_agent` function's docstring included a `Note` section with developer guidance about using `TransferToAgentTool` instead. Since this docstring is sent to the model as the tool description on every invocation, the extra text wastes input tokens and increases hallucination risk.

## Before

```
description: "Transfer the question to another agent. This tool hands off control to another agent when it's more suitable to answer the user's question according to the agent's description. Note: For most use cases, you should use TransferToAgentTool instead of this function directly. TransferToAgentTool provides additional enum constraints that prevent LLMs from hallucinating invalid agent names. Args: agent_name: the agent name to transfer to."
```

## After

```
description: "Transfer the question to another agent. Use this tool to hand off control to another agent that is more suitable to answer the user's question according to the agent's description. Args: agent_name: the agent name to transfer to."
```

## Changes

- `src/google/adk/tools/transfer_to_agent_tool.py`: Removed the `Note` section from the docstring, keeping only the model-facing description and Args.

## Testing

All 9 existing tests in `test_transfer_to_agent_tool.py` pass, including `test_transfer_to_agent_tool_preserves_description` which validates the tool description content.

Co-authored-by: Bo Yang <ybo@google.com>
COPYBARA_INTEGRATE_REVIEW=#4617 from stakeswky:fix/transfer-to-agent-docstring-cleanup 4c78693
PiperOrigin-RevId: 930320693

Author: 2 people

src/google/adk/tools/transfer_to_agent_tool.py

@@ -23,17 +23,16 @@
 from .tool_context import ToolContext
 
 
+# Note:
+#   For most use cases, you should use TransferToAgentTool instead of this
+#   function directly. TransferToAgentTool provides additional enum constraints
+#   that prevent LLMs from hallucinating invalid agent names.
 def transfer_to_agent(agent_name: str, tool_context: ToolContext) -> None:
   """Transfer the question to another agent.
 
-  This tool hands off control to another agent when it's more suitable to
+  Use this tool to hand off control to another agent that is more suitable to
   answer the user's question according to the agent's description.
 
-  Note:
-    For most use cases, you should use TransferToAgentTool instead of this
-    function directly. TransferToAgentTool provides additional enum constraints
-    that prevent LLMs from hallucinating invalid agent names.
-
   Args:
     agent_name: the agent name to transfer to.
   """