History save fails for app-local S7 `Content` subclasses: "Unable to find the S7 class"

When a chat contains ellmer::Content subclasses defined in a Shiny app's R/ files (i.e., not in a package), saving history fails with:

Warning: Could not save conversation
Caused by error in `FUN()`:
! Unable to find the S7 class: `ContentPromptStyle`.

Repro

library(S7)
library(ellmer)
# Shiny sources R/ helpers into new.env(parent = globalenv()), not globalenv()
app_env <- new.env(parent = globalenv())
eval(quote({
  ContentPromptStyle <- new_class(
    "ContentPromptStyle",
    parent = ellmer::ContentText,
    properties = list(slug = new_property(class_character, default = "friendly"))
  )
}), envir = app_env)
turn <- with(app_env, Turn("user", list(ContentPromptStyle(text = "hi"))))
recorded <- ellmer::contents_record(turn)
# shinychat replays recorded turns at save time from its own namespace frame,
# e.g. extend_record_linear() -> lapply(turns, ellmer::contents_replay, tools)
f <- eval(
  quote(function(x) lapply(x, ellmer::contents_replay, tools = list())),
  new.env(parent = asNamespace("shinychat"))
)
f(list(recorded))
#> Error: Unable to find the S7 class: `ContentPromptStyle`.

What's happening

1. ellmer::contents_record() serializes a non-package class as a bare name ("ContentPromptStyle", no pkg:: prefix).
2. shinychat:::extend_record_linear() and shinychat:::derive_node_ui_from_turns() call ellmer::contents_replay() without .envir, so it defaults to parent.frame() — a frame inside shinychat's namespace.
3. contents_replay() resolves bare class names with get0(name, envir, inherits = TRUE). The namespace parent chain reaches globalenv() but never descends into the app environment where Shiny sourced the class, so the lookup fails.

The current workaround is for apps to assign("MyClass", MyClass, envir = globalenv()), which works but is non-obvious and easy to get wrong.

Possible directions — not sure what the right fix is

• Capture the app's environment at chat_server() setup (where the client/turns live) and pass it as .envir to contents_replay().
• Expose a replay environment on history_options() so apps defining custom content classes can say where they live.
• At minimum, document that custom Content subclasses must be resolvable from globalenv() (or a package namespace) for history to work.

Start with shinychat:::extend_record_linear() and shinychat:::derive_node_ui_from_turns(), then inspect ellmer::contents_replay() and its .envir lookup. Reproduce the failure using an S7 Content subclass in a child app environment, and determine how the replay context should be supplied. Done means history replay succeeds for app-local classes without requiring assignment to globalenv(), with regression coverage.