fix: tolerate NULL state$values in onBookmark callback

[cpsievert]

Problem

With chat history enabled, submitting a prompt can produce:

Warning: Error in utils::modifyList: is.list(x) is not TRUE
  ...
  131: state$onSave
   99: session$doBookmark
   98: self$on_response_saved
   97: controller$on_response

shinychat's history save triggers a Shiny bookmark (session$doBookmark()) after each response. In that context the onBookmark callback's state$values can be NULL, but utils::modifyList(NULL, ...) fails because modifyList() requires a list as its first argument.

Repro

# inside mod_server, the onBookmark callback does:
shiny::onBookmark(function(state) {
  state$values <- utils::modifyList(state$values, build_state_snapshot())
})

# when history triggers the bookmark, this can arrive as:
state$values <- NULL
utils::modifyList(NULL, list(querychat_tables = ...))
# Error: is.list(x) is not TRUE

Observed in querychat_app(penguins) with default (file-based) history after submitting a prompt.

Fix

Coalesce state$values to an empty list before merging the state snapshot:

state$values <- utils::modifyList(state$values %||% list(), build_state_snapshot())

Includes a regression test that drives mod_server via testServer and invokes the captured onBookmark callback with state$values <- NULL.