Add Google ADK agent example connecting to ToolHive vMCP

User Story

As a ToolHive user attempting to build my own agent service with Google ADK, I have no
reference implementation showing how to securely connect a Google ADK agent to MCP
servers managed by ToolHive. Without this, I have to figure out the integration from
scratch.

Description

Create a self-contained Google ADK agent example in the toolhive repository that
demonstrates connecting a Google ADK agent to a ToolHive-managed vMCP endpoint with
Okta OIDC auth. Google ADK is gaining traction for building production agent services;
this example shows how ToolHive fits into that ecosystem.

Requirements

1. Create examples/agents/google-adk/ directory with a runnable Python example
2. Demonstrate how to initiate the SSO flow from the agent service to authenticate to a vMCP
3. Show the agent invoking at least one MCP tool through the vMCP
4. Include a README.md with prerequisites, setup steps, and how to run
5. Keep dependencies minimal and pinned

Acceptance Criteria

• Example is checked into examples/agents/google-adk/ in the toolhive repo
• A user can follow the README and run the example against their own vMCP
• The example demonstrates successful MCP tool invocation through the vMCP
• The SSO flow is initiated from within the agent service
• README explains the ADK agent structure and how MCP is wired in

Create the self-contained runnable example under examples/agents/google-adk/ and include its README.md. Start by defining the Python agent, its ToolHive-managed vMCP connection, and the Okta OIDC SSO flow; done means a user can follow the prerequisites and setup steps, run the agent against their own vMCP, and invoke an MCP tool successfully.