fix(pkg-r): make viz bookmark restore survive serialization

[gadenbuie]

Relates to posit-dev/shinychat#261

Problem

With bookmarking enabled, restoring a session whose chat history contains a querychat_visualize tool call breaks on reload. Two distinct failures stack up:

1. could not find function "tag_require_client_side" — the visualize tool embeds live bslib/htmltools tags (notably a bslib::input_code_editor() in the footer) in the ContentToolResult extra$display. shinychat bookmarks chat turns with jsonlite::serializeJSON() and restores them with unserializeJSON(), which rebuilds embedded closures from deparsed text and drops their environment. The render hooks on those tags can then no longer reach bslib internals, so re-rendering the restored tool card throws. (Root-caused in detail in Bookmark restore drops render-hook environments: tool-result displays with live bslib/htmltools tags fail to re-render shinychat#261.)

2. $ operator is invalid for atomic vectors — a URL bookmark round-trips querychat_viz_widgets through jsonlite, which simplifies the JSON array of objects to a data.frame. restore_viz_widgets() then iterated its columns (atomic vectors), so the transcript chart never re-rendered.

Changes

• freeze_tags() (new, utils-html.R) resolves a tag's render hooks to inert HTML + html_dependency objects up front — while the hooks can still reach those internals — so the display survives the bookmark serialize/deserialize round-trip. Applied to the visualize tool's display$html and display$footer. The frozen footer carries the prism-code-editor / bslib-code-editor dependencies through the round-trip, so the editor still hydrates on restore (no explicit dependency injection needed).
• restore_record_list() (new, querychat_module.R) rebuilds the list-of-lists shape from the URL-decoded data.frame row by row (dropping absent NA fields); the querychat_viz_widgets restore is wrapped with it.

Verification

Reproduced and confirmed fixed with this app:

library(shiny)
qc <- querychat::QueryChat$new(
  mtcars,
  tools = "visualize",
  greeting = "Ask me about the `mtcars` dataset.",
  client = ellmer::chat_openai(model = "gpt-5.4-nano")
)
shiny::enableBookmarking("url")
qc$app_obj()

Submit a prompt that produces a chart (e.g. "plot how many cars have 6 or 8 cylinders"), wait for it to render, then reload the page to restore the bookmark. Before: both errors above (the chat transcript fails to re-render). After: no errors, and the transcript chart and code editor both re-render on restore.

Notes

• The underlying serialization fragility is a shinychat concern (Bookmark restore drops render-hook environments: tool-result displays with live bslib/htmltools tags fail to re-render shinychat#261); this PR is a targeted querychat-side workaround.
• Test suite not yet run.