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/assistant_tools/Cargo.toml

@@ -20,4 +20,3 @@ project.workspace = true
 schemars.workspace = true
 serde.workspace = true
 serde_json.workspace = true
-workspace.workspace = true