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/scripting_tool_description.txt

@@ -1,22 +0,0 @@
-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.
-
-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.
-
-Also, I'm going to be putting this Lua script into JSON, so please don't use
-Lua's double quote syntax for string literals - use one of Lua's other syntaxes
-for string literals, so I don't have to escape the double quotes.
-
-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).