Inline assistant v2 (#21828)

This is behind the Assistant v2 feature flag. As @maxdeviant and I
discussed, the state is currently decoupled from the Assistant Panel's
state, although in the future we plan to introduce a way to refer to
conversations from the panel. Also, we're intentionally duplicating some
code with the v2 panel right now; the plan is to do a future PR to make
them share code more.


https://github.com/user-attachments/assets/bb163bd3-a02d-4a91-8f8f-2a8e60acbc34

It doesn't include the terminal inline assistant, which will be in a
separate PR.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -406,7 +406,12 @@ fn main() {
             stdout_is_a_pty(),
             cx,
         );
-        assistant2::init(cx);
+        assistant2::init(
+            app_state.fs.clone(),
+            app_state.client.clone(),
+            stdout_is_a_pty(),
+            cx,
+        );
         assistant_tools::init(cx);
         repl::init(
             app_state.fs.clone(),