fix: set example seller price explicitly

Author: Matias

python/examples/src/examples/a2a/seller/adk/agent.py

@@ -10,7 +10,9 @@
 root_agent = Agent(
     name="facts_agent",
     before_agent_callback=make_x402_before_agent_callback(
-        pay_to_address=os.environ["EXAMPLES_A2A_SELLER__PAY_TO_ADDRESS"]
+        price="$0.001",
+        network="base-sepolia",
+        pay_to_address=os.environ["EXAMPLES_A2A_SELLER__PAY_TO_ADDRESS"],
     ),
     model="gemini-2.5-flash-lite",
     description=("Agent to give interesting facts."),