assistant_tool: Decouple Tool from Workspace (#26309)

This PR decouples the `Tool` trait from the `Workspace` (and from the
UI, in general).

`Tool::run` now takes a `WeakEntity<Project>` instead of a
`WeakEntity<Workspace>` and a `Window`.

Release Notes:

- N/A

crates/context_server/Cargo.toml

@@ -31,4 +31,3 @@ settings.workspace = true
 smol.workspace = true
 url = { workspace = true, features = ["serde"] }
 util.workspace = true
-workspace.workspace = true