docs(genai-tools): Set the reactive value directly from a tool

[jat255]

This updates the tool-calling docs for the deprecation of reactive.lock(). A tool now sets a reactive value directly, with no lock and no flush.

Do not merge until a py-shiny release includes posit-dev/py-shiny#2508 and posit-dev/py-shiny#2520. In the released Shiny, a set() from a background task does not schedule a flush, so this example would not update the dashboard.

Summary

• The update_dashboard() example in docs/genai-tools.qmd calls current_query.set(query) and nothing else.
• The "Reactive locking" callout is replaced. The new callout says that a tool in a non-blocking stream can set a reactive value directly, because setting a value schedules a reactive update.

Review Notes

• This is the only page in the site that uses reactive.lock() or reactive.flush(). The API reference pages come from py-shiny's docstrings, and those are updated in feat(reactive): Deprecate reactive.lock() py-shiny#2520.
• The callout no longer mentions blocking streams, because the same code is now correct in both kinds of stream.

Testing

I did not render the site. The change is a code block and a callout in one .qmd file.

Refs posit-dev/py-shiny#698, posit-dev/py-shiny#2508, posit-dev/py-shiny#2520