Remove the nested Pane from the assistant

Since we don't want tabs, I think it would be better to render the toolbar
for ourselves directly and handle switching between conversations.

Co-Authored-By: Julia Risley <julia@zed.dev>

crates/ai/Cargo.toml

@@ -34,3 +34,4 @@ tiktoken-rs = "0.4"
 
 [dev-dependencies]
 editor = { path = "../editor", features = ["test-support"] }
+project = { path = "../project", features = ["test-support"] }