assistant: Make scripting a first-class concept instead of a tool (#26338)

This PR makes refactors the scripting functionality to be a first-class
concept of the assistant instead of a generic tool, which will allow us
to build a more customized experience.

- The tool prompt has been slightly tweaked and is now included as a
system message in all conversations. I'm getting decent results, but now
that it isn't in the tools framework, it will probably require more
refining.

- The model will now include an `<eval ...>` tag at the end of the
message with the script. We parse this tag incrementally as it streams
in so that we can indicate that we are generating a script before we see
the closing `</eval>` tag. Later, this will help us interpret the script
as it arrives also.

- Threads now hold a `ScriptSession` entity which manages the state of
all scripts (from parsing to exited) in a centralized way, and will
later collect all script operations so they can be displayed in the UI.

- `script_tool` has been renamed to `assistant_scripting` 

- Script source now opens in a regular read-only buffer  

Note: We still need to handle persistence properly

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/scripting_tool/src/sandbox_preamble.lua

@@ -1,40 +0,0 @@
----@diagnostic disable: undefined-global
-
--- Create a sandbox environment
-local sandbox = {}
-
--- Allow access to standard libraries (safe subset)
-sandbox.string = string
-sandbox.table = table
-sandbox.math = math
-sandbox.print = sb_print
-sandbox.type = type
-sandbox.tostring = tostring
-sandbox.tonumber = tonumber
-sandbox.pairs = pairs
-sandbox.ipairs = ipairs
-sandbox.search = search
-
--- Create a sandboxed version of LuaFileIO
-local io = {}
-
--- File functions
-io.open = sb_io_open
-
--- Add the sandboxed io library to the sandbox environment
-sandbox.io = io
-
-
--- Load the script with the sandbox environment
-local user_script_fn, err = load(user_script, nil, "t", sandbox)
-
-if not user_script_fn then
-  error("Failed to load user script: " .. tostring(err))
-end
-
--- Execute the user script within the sandbox
-local success, result = pcall(user_script_fn)
-
-if not success then
-  error("Error executing user script: " .. tostring(result))
-end