Reapply "Use Project instead of Workspace in ContextStore (#28402)" (#28441)

Motivation for this change is to use `ContextStore` in headless
assistant, which requires it to not depend on UI entities like
`Workspace`.

This reapplies a change that was revert was in #28428, and fixes the panic.

Release Notes:

- N/A

crates/agent/src/context_picker/completion_provider.rs

@@ -867,7 +867,7 @@ mod tests {
                 .expect("Opened test file wasn't an editor")
         });
 
-        let context_store = cx.new(|_| ContextStore::new(workspace.downgrade(), None));
+        let context_store = cx.new(|_| ContextStore::new(project.downgrade(), None));
 
         let editor_entity = editor.downgrade();
         editor.update_in(&mut cx, |editor, window, cx| {