feat: Add enum constraint to agent_name for transfer_to_agent

[qieqieplus]

Current implementation of transfer_to_agent doesn't enforce strict constraints on agent names,
We could use JSON Schema's enum definition to implement stricter constraints:

Before

[{
	"type": "function",
	"function": {
		"name": "transfer_to_agent",
		"description": "Transfer the question to another agent.\n\nThis tool hands off control to another agent when it's more suitable to answer the user's question according to the agent's description.\n\nArgs:\n  agent_name: the agent name to transfer to.\n",
		"parameters": {
			"type": "object",
			"properties": {
				"agent_name": {
					"type": "string"
				}
			}
		}
	}
}]

After

[{
	"type": "function",
	"function": {
		"name": "transfer_to_agent",
		"description": "Transfer the question to another agent.\n\nThis tool hands off control to another agent when it's more suitable to answer the user's question according to the agent's description.\n\nArgs:\n  agent_name: the agent name to transfer to.\n",
		"parameters": {
			"type": "object",
			"properties": {
				"agent_name": {
					"type": "string",
					"enum": ["agent_a", "agent_b", "agent_c"]
				}
			}
		}
	}
}]

[adk-bot]

Response from ADK Triaging Agent

Hello @qieqieplus, thank you for your contribution!

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[hangfei]

Could you resolve the conflicts so we can merge?

[qieqieplus]

Could you resolve the conflicts so we can merge?

Done.
I noticed there is a update of agent_transfer.py,
with a new reference to _TRANSFER_TO_AGENT_FUNCTION_NAME,
so I updated this during resolution to match the refactoring.

[adk-bot]

Thank you @qieqieplus for your contribution! 🎉

Your changes have been successfully imported and merged via Copybara in commit 4a42d0d.

Closing this PR as the changes are now in the main branch.