azureai direct a2a endpoint support (microsoft#3127)

Author: giles17

python/samples/getting_started/agents/azure_ai/azure_ai_with_agent_to_agent.py

@@ -14,22 +14,31 @@
 Prerequisites:
 1. Set AZURE_AI_PROJECT_ENDPOINT and AZURE_AI_MODEL_DEPLOYMENT_NAME environment variables.
 2. Ensure you have an A2A connection configured in your Azure AI project
-    and set A2A_PROJECT_CONNECTION_ID environment variable.
+   and set A2A_PROJECT_CONNECTION_ID environment variable.
+3. (Optional) A2A_ENDPOINT - If the connection is missing target (e.g., "Custom keys" type),
+   set the A2A endpoint URL directly.
 """
 
 
 async def main() -> None:
+    # Configure A2A tool with connection ID
+    a2a_tool = {
+        "type": "a2a_preview",
+        "project_connection_id": os.environ["A2A_PROJECT_CONNECTION_ID"],
+    }
+    
+    # If the connection is missing a target, we need to set the A2A endpoint URL
+    if os.environ.get("A2A_ENDPOINT"):
+        a2a_tool["base_url"] = os.environ["A2A_ENDPOINT"]
+    
     async with (
         AzureCliCredential() as credential,
         AzureAIClient(credential=credential).create_agent(
             name="MyA2AAgent",
             instructions="""You are a helpful assistant that can communicate with other agents.
             Use the A2A tool when you need to interact with other agents to complete tasks
             or gather information from specialized agents.""",
-            tools={
-                "type": "a2a_preview",
-                "project_connection_id": os.environ["A2A_PROJECT_CONNECTION_ID"],
-            },
+            tools=a2a_tool,
         ) as agent,
     ):
         query = "What can the secondary agent do?"