chore: update Chat API reference example (#2050)

Co-authored-by: Copilot <[email protected]>

Author: cpsievert

shiny/api-examples/Chat/app-core.py

@@ -3,21 +3,24 @@
 app_ui = ui.page_fillable(
     ui.panel_title("Hello Shiny Chat"),
     ui.chat_ui("chat"),
-    fillable_mobile=True,
-)
-
-# Create a welcome message
-welcome = """
+    ui.chat_ui(
+        "chat",
+        messages=[
+            """
 Hi! This is a simple Shiny `Chat` UI. Enter a message below and I will
 simply repeat it back to you.
 
 To learn more about chatbots and how to build them with Shiny, check out
 [the documentation](https://shiny.posit.co/py/docs/genai-chatbots.html).
 """
+        ],
+    ),
+    fillable_mobile=True,
+)
 
 
 def server(input, output, session):
-    chat = ui.Chat(id="chat", messages=[welcome])
+    chat = ui.Chat(id="chat")
 
     # Define a callback to run when the user submits a message
     @chat.on_user_submit

shiny/api-examples/Chat/app-express.py

@@ -7,24 +7,22 @@
     fillable_mobile=True,
 )
 
-# Create a welcome message
-welcome = """
+# Create a chat instance
+chat = ui.Chat(id="chat")
+
+# Display it, with a startup message
+chat.ui(
+    messages=[
+        """
 Hi! This is a simple Shiny `Chat` UI. Enter a message below and I will
 simply repeat it back to you.
 
 To learn more about chatbots and how to build them with Shiny, check out
 [the documentation](https://shiny.posit.co/py/docs/genai-chatbots.html).
 """
-
-# Create a chat instance
-chat = ui.Chat(
-    id="chat",
-    messages=[welcome],
+    ],
 )
 
-# Display it
-chat.ui()
-
 
 # Define a callback to run when the user submits a message
 @chat.on_user_submit