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>

Cargo.lock

@@ -11916,6 +11916,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "assistant_tool",
+ "collections",
  "futures 0.3.31",
  "gpui",
  "mlua",
@@ -11925,7 +11926,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
- "smol",
+ "settings",
  "util",
  "workspace",
 ]