Describe the bug
After upgrading to ADK v1.16, requests that stream tool outputs show a noticeable latency increase. Tracing reveals that McpToolset.get_tools() is invoked multiple times during a single streamed interaction, repeatedly reconstructing the same tool list. This results in unnecessary network/IPC work and object creation, compounding latency.

To Reproduce
Steps to reproduce the behavior:

1. Install 'google-adk==1.16.*' (or build from main including v1.16 changes).
2. Configure an MCP server with a stable set of tools (no dynamic changes at runtime).
3. Run a streaming generation that calls MCP tools (e.g., tool-calling with partial deltas).
4. Observe logs/trace: get_tools() is called several times per request; end-to-end latency increases vs. pre-1.16.

Expected behavior

• get_tools() should only resolve/build the tool list once per McpToolset instance (or per session), and reuse it when the set of available tools is unchanged.
• Streaming should not incur repeated, identical tool discovery costs.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. macOS, Linux, Windows]
• Python version(python -V):
• ADK version(pip show google-adk):

Model Information:

• Are you using LiteLLM: Yes/No
• Which model is being used(e.g. gemini-2.5-pro)

Additional context
Add any other context about the problem here.