docs: annotate test token in example as public

Add clarifying comments to indicate the auth_token in the example
is a public test token, not a secret. This token is already used
throughout the codebase in README.md and test files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bokelley

examples/simple_api_demo.py

@@ -85,12 +85,13 @@ async def demo_production_client():
     # Create a production client
     from adcp import ADCPClient, AgentConfig, Protocol
 
+    # Note: This is a public test token for demo purposes
     client = ADCPClient(
         AgentConfig(
             id="my-agent",
             agent_uri="https://test-agent.adcontextprotocol.org/mcp/",
             protocol=Protocol.MCP,
-            auth_token="1v8tAhASaUYYp4odoQ1PnMpdqNaMiTrCRqYo9OJp6IQ",
+            auth_token="1v8tAhASaUYYp4odoQ1PnMpdqNaMiTrCRqYo9OJp6IQ",  # Public test token
         )
     )