Extract a Session struct to hold state about a given thread's scripting session (#26282)

We're still recreating a session for every tool call, but the idea is to
have a long-lived `Session` per assistant thread.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/scripting_tool/Cargo.toml

@@ -15,6 +15,7 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 assistant_tool.workspace = true
+collections.workspace = true
 futures.workspace = true
 gpui.workspace = true
 mlua.workspace = true
@@ -24,6 +25,13 @@ regex.workspace = true
 schemars.workspace = true
 serde.workspace = true
 serde_json.workspace = true
-smol.workspace = true
+settings.workspace = true
 util.workspace = true
 workspace.workspace = true
+
+[dev-dependencies]
+collections = { workspace = true, features = ["test-support"] }
+gpui = { workspace = true, features = ["test-support"] }
+project = { workspace = true, features = ["test-support"] }
+settings = { workspace = true, features = ["test-support"] }
+workspace = { workspace = true, features = ["test-support"] }