fix: remove developer note from transfer_to_agent tool description

The docstring of transfer_to_agent() included a 'Note:' section with
developer-facing advice about preferring TransferToAgentTool. Since
FunctionTool uses the full docstring as the tool description sent to
the model on every invocation, this note was consuming input tokens
and adding irrelevant information to the model context.

Move the note to an inline comment in the function body so it remains
visible to developers but is not included in the tool description.

Fixes #4615

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giulio-leone

src/google/adk/tools/transfer_to_agent_tool.py

@@ -29,14 +29,12 @@ def transfer_to_agent(agent_name: str, tool_context: ToolContext) -> None:
   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.
   """
+  # Developer note: For most use cases, prefer TransferToAgentTool over
+  # calling this function directly. TransferToAgentTool adds enum
+  # constraints that prevent LLMs from hallucinating invalid agent names.
   tool_context.actions.transfer_to_agent = agent_name