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_store.rs

@@ -8,11 +8,10 @@ use futures::future::join_all;
 use futures::{self, Future, FutureExt, future};
 use gpui::{App, AppContext as _, Context, Entity, SharedString, Task, WeakEntity};
 use language::{Buffer, File};
-use project::{ProjectItem, ProjectPath, Worktree};
+use project::{Project, ProjectItem, ProjectPath, Worktree};
 use rope::Rope;
 use text::{Anchor, BufferId, OffsetRangeExt};
 use util::{ResultExt as _, maybe};
-use workspace::Workspace;
 
 use crate::ThreadStore;
 use crate::context::{
@@ -23,7 +22,7 @@ use crate::context_strip::SuggestedContext;
 use crate::thread::{Thread, ThreadId};
 
 pub struct ContextStore {
-    workspace: WeakEntity<Workspace>,
+    project: WeakEntity<Project>,
     context: Vec<AssistantContext>,
     thread_store: Option<WeakEntity<ThreadStore>>,
     // TODO: If an EntityId is used for all context types (like BufferId), can remove ContextId.
@@ -40,11 +39,11 @@ pub struct ContextStore {
 
 impl ContextStore {
     pub fn new(
-        workspace: WeakEntity<Workspace>,
+        project: WeakEntity<Project>,
         thread_store: Option<WeakEntity<ThreadStore>>,
     ) -> Self {
         Self {
-            workspace,
+            project,
             thread_store,
             context: Vec::new(),
             next_context_id: ContextId(0),
@@ -81,12 +80,7 @@ impl ContextStore {
         remove_if_exists: bool,
         cx: &mut Context<Self>,
     ) -> Task<Result<()>> {
-        let workspace = self.workspace.clone();
-
-        let Some(project) = workspace
-            .upgrade()
-            .map(|workspace| workspace.read(cx).project().clone())
-        else {
+        let Some(project) = self.project.upgrade() else {
             return Task::ready(Err(anyhow!("failed to read project")));
         };
 
@@ -161,11 +155,7 @@ impl ContextStore {
         remove_if_exists: bool,
         cx: &mut Context<Self>,
     ) -> Task<Result<()>> {
-        let workspace = self.workspace.clone();
-        let Some(project) = workspace
-            .upgrade()
-            .map(|workspace| workspace.read(cx).project().clone())
-        else {
+        let Some(project) = self.project.upgrade() else {
             return Task::ready(Err(anyhow!("failed to read project")));
         };