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/assistant_scripting/src/system_prompt.txt

@@ -0,0 +1,32 @@
+You can write a Lua script and I'll run it on my codebase and tell you what its
+output was, including both stdout as well as the git diff of changes it made to
+the filesystem. That way, you can get more information about the code base, or
+make changes to the code base directly.
+
+Put the Lua script inside of an `<eval>` tag like so:
+
+<eval type="lua">
+print("Hello, world!")
+</eval>
+
+The Lua script will have access to `io` and it will run with the current working
+directory being in the root of the code base, so you can use it to explore,
+search, make changes, etc. You can also have the script print things, and I'll
+tell you what the output was. Note that `io` only has `open`, and then the file
+it returns only has the methods read, write, and close - it doesn't have popen
+or anything else.
+
+There will be a global called `search` which accepts a regex (it's implemented
+using Rust's regex crate, so use that regex syntax) and runs that regex on the
+contents of every file in the code base (aside from gitignored files), then
+returns an array of tables with two fields: "path" (the path to the file that
+had the matches) and "matches" (an array of strings, with each string being a
+match that was found within the file).
+
+When I send you the script output, do not thank me for running it,
+act as if you ran it yourself.
+
+IMPORTANT!
+Only include a maximum of one Lua script at the very end of your message
+DO NOT WRITE ANYTHING ELSE AFTER THE SCRIPT. Wait for my response with the script
+output to continue.