feat!(pkg-r): add chat_server(), deprecate chat_mod_server() and chat_mod_ui()

[cpsievert]

Summary

chat_server() replaces chat_mod_server() as the primary way to wire up server-side chat logic.

The original module-based API (chat_mod_ui() / chat_mod_server()) was more structure than the use case needs. The goal of this API is simple: you hand us an ellmer client and we hook up all the server logic most people want — streaming, cancellation, bookmarking, etc. A full moduleServer() scope isn't required to deliver that. Furthermore, since part of the ID namespace is private/implicit, downstream logic that depends on the chat's ID (e.g., slash command JS events) need workarounds to work in the way users might expect. This change also better aligns the R package with the Python API where no modules are needed to get the "batteries included" experience.

chat_server() does the same job but runs directly in the caller's session scope. If you're already inside a moduleServer(), you pass that session in — no extra nesting, no doubled namespaces.

What changed

• chat_server(id, client, ..., session) — new exported function with the same logic as chat_mod_server(), but runs in the provided session scope rather than creating its own. Takes an explicit session parameter (defaults to getDefaultReactiveDomain()).
• chat_mod_server() — soft-deprecated (0.5.0). Now a thin wrapper around chat_server().
• chat_mod_ui() — soft-deprecated (0.5.0). Replace with chat_ui(NS(id, "chat"), ...) in your module UI.

Migration

The simplest case — no module needed at all:

# Before (forced you into a module)
ui <- fluidPage(chat_mod_ui("chat"))
server <- function(input, output, session) {
  chat_mod_server("chat", client)
}

# After
ui <- fluidPage(chat_ui("chat"))
server <- function(input, output, session) {
  chat_server("chat", client)
}

If you're embedding chat inside an existing module, pass that module's session explicitly:

ui <- function(id) {
  ns <- NS(id)
  chat_ui(ns("chat"))
}
server <- function(id, client) {
  shiny::moduleServer(id, function(input, output, session) {
    chat_server("chat", client, session = session)
  })
}

Existing code using chat_mod_server() continues to work with a deprecation notice.

[gadenbuie]

Looks good, thanks!