agent: Use default prompts from prompt library in system prompt (#28915)

Related to #28490.

- Default prompts from the prompt library are now included as "user
rules" in the system prompt.
- Presence of these user rules is shown at the beginning of the thread
in the UI.
_ Now uses an `Entity<PromptStore>` instead of an `Arc<PromptStore>`.
Motivation for this is emitting a `PromptsUpdatedEvent`.
- Now disallows concurrent reloading of the system prompt. Before this
change it was possible for reloads to race.

Release Notes:

- agent: Added support for including default prompts from the Prompt
Library as "user rules" in the system prompt.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent/src/thread.rs

@@ -4,7 +4,7 @@ use std::ops::Range;
 use std::sync::Arc;
 use std::time::Instant;
 
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Result, anyhow};
 use assistant_settings::AssistantSettings;
 use assistant_tool::{ActionLog, AnyToolCard, Tool, ToolWorkingSet};
 use chrono::{DateTime, Utc};
@@ -939,7 +939,7 @@ impl Thread {
     pub fn to_completion_request(
         &self,
         request_kind: RequestKind,
-        cx: &App,
+        cx: &mut Context<Self>,
     ) -> LanguageModelRequest {
         let mut request = LanguageModelRequest {
             messages: vec![],
@@ -949,20 +949,33 @@ impl Thread {
         };
 
         if let Some(project_context) = self.project_context.borrow().as_ref() {
-            if let Some(system_prompt) = self
+            match self
                 .prompt_builder
                 .generate_assistant_system_prompt(project_context)
-                .context("failed to generate assistant system prompt")
-                .log_err()
             {
-                request.messages.push(LanguageModelRequestMessage {
-                    role: Role::System,
-                    content: vec![MessageContent::Text(system_prompt)],
-                    cache: true,
-                });
+                Err(err) => {
+                    let message = format!("{err:?}").into();
+                    log::error!("{message}");
+                    cx.emit(ThreadEvent::ShowError(ThreadError::Message {
+                        header: "Error generating system prompt".into(),
+                        message,
+                    }));
+                }
+                Ok(system_prompt) => {
+                    request.messages.push(LanguageModelRequestMessage {
+                        role: Role::System,
+                        content: vec![MessageContent::Text(system_prompt)],
+                        cache: true,
+                    });
+                }
             }
         } else {
-            log::error!("project_context not set.")
+            let message = "Context for system prompt unexpectedly not ready.".into();
+            log::error!("{message}");
+            cx.emit(ThreadEvent::ShowError(ThreadError::Message {
+                header: "Error generating system prompt".into(),
+                message,
+            }));
         }
 
         for message in &self.messages {
@@ -2163,7 +2176,7 @@ fn main() {{
         assert_eq!(message.context, expected_context);
 
         // Check message in request
-        let request = thread.read_with(cx, |thread, cx| {
+        let request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2255,7 +2268,7 @@ fn main() {{
         assert!(message3.context.contains("file3.rs"));
 
         // Check entire request to make sure all contexts are properly included
-        let request = thread.read_with(cx, |thread, cx| {
+        let request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2307,7 +2320,7 @@ fn main() {{
         assert_eq!(message.context, "");
 
         // Check message in request
-        let request = thread.read_with(cx, |thread, cx| {
+        let request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2327,7 +2340,7 @@ fn main() {{
         assert_eq!(message2.context, "");
 
         // Check that both messages appear in the request
-        let request = thread.read_with(cx, |thread, cx| {
+        let request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2369,7 +2382,7 @@ fn main() {{
         });
 
         // Create a request and check that it doesn't have a stale buffer warning yet
-        let initial_request = thread.read_with(cx, |thread, cx| {
+        let initial_request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2399,7 +2412,7 @@ fn main() {{
         });
 
         // Create a new request and check for the stale buffer warning
-        let new_request = thread.read_with(cx, |thread, cx| {
+        let new_request = thread.update(cx, |thread, cx| {
             thread.to_completion_request(RequestKind::Chat, cx)
         });
 
@@ -2428,6 +2441,7 @@ fn main() {{
             language::init(cx);
             Project::init_settings(cx);
             AssistantSettings::register(cx);
+            prompt_store::init(cx);
             thread_store::init(cx);
             workspace::init_settings(cx);
             ThemeSettings::register(cx);
@@ -2467,7 +2481,8 @@ fn main() {{
                     cx,
                 )
             })
-            .await;
+            .await
+            .unwrap();
 
         let thread = thread_store.update(cx, |store, cx| store.create_thread(cx));
         let context_store = cx.new(|_cx| ContextStore::new(project.downgrade(), None));