Support Shiny UI inside of chat_append() and markdown_stream()

[cpsievert]

This PR adds support for the content of a message provided to chat_append() or markdown_stream() to include Shiny UI, and moreover, if that UI includes output or input bindings, they are bound to the server session.

This means, among other things, you can now display things like {htmlwidgets} and collect user input from things like a select input. Here are a couple basic examples:

Output example

library(shiny)
library(shinychat)
library(bslib)
library(plotly)

welcome <- list(
  list(
    "**Hello! Here is a statically rendered widget**",
    plot_ly(x = 1, y = 1, mode = "text", text = "Statically rendered widget")
  ),
  list(
    "**And here is an output binding**",
    plotlyOutput("plt")
  )
)

ui <- page_fillable(
  title = "Hello widgets in chat",
  fillable_mobile = TRUE,
  chat_ui("chat", messages = welcome)
)

server <- function(input, output, session) {

  observeEvent(input$chat_user_input, {
    chat_append("chat", paste("You said:", input$chat_user_input))
  })

  output$plt <- renderPlotly({
    plot_ly(x = 1, y = 1, mode = "text", text = "Dynamically rendered widget")
  })

}


shinyApp(ui, server)

Input example

library(ellmer)
library(shiny)
library(shinychat)
library(bslib)
library(plotly)


welcome <- list(
  list(
    "**Hello! How would you like me to respond?**",
    selectInput("tone", "", choices = c("Happy", "Sad"))
  )
)

ui <- page_fillable(
  title = "Hello input bindings in chat",
  fillable_mobile = TRUE,
  chat_ui("chat", messages = welcome)
)

server <- function(input, output, session) {

  chat <- chat_ollama(model = "llama3.2")

  observeEvent(input$chat_user_input, {
    stream <- chat$stream_async(input$chat_user_input)
    chat_append("chat", stream)
  })

  observeEvent(input$tone, {
    chat$set_system_prompt(paste("You are a terse and", input$tone, "assistant."))
  })

}


shinyApp(ui, server)

TODO

• Merge feat(Chat, MarkdownStream): Support Shiny UI inside of message content py-shiny#1868
• Documentation

[gadenbuie]

Looks good! I just left a few very minor comments