assistant2: Store deduped context on the Thread (#22781)

This PR is a small refactoring in advance of some other changes.

Previously we were storing the whole `Context` associated with each
message. However, it's likely that multiple messages may end up using
the same context.

We now store the deduped context in a separate collection and refer to
it from each message by its `ContextId`.

Release Notes:

- N/A

crates/assistant2/src/thread.rs

@@ -17,7 +17,7 @@ use serde::{Deserialize, Serialize};
 use util::{post_inc, TryFutureExt as _};
 use uuid::Uuid;
 
-use crate::context::{attach_context_to_message, Context};
+use crate::context::{attach_context_to_message, Context, ContextId};
 
 #[derive(Debug, Clone, Copy)]
 pub enum RequestKind {
@@ -64,7 +64,8 @@ pub struct Thread {
     pending_summary: Task<Option<()>>,
     messages: Vec<Message>,
     next_message_id: MessageId,
-    context_by_message: HashMap<MessageId, Vec<Context>>,
+    context: HashMap<ContextId, Context>,
+    context_by_message: HashMap<MessageId, Vec<ContextId>>,
     completion_count: usize,
     pending_completions: Vec<PendingCompletion>,
     tools: Arc<ToolWorkingSet>,
@@ -82,6 +83,7 @@ impl Thread {
             pending_summary: Task::ready(None),
             messages: Vec::new(),
             next_message_id: MessageId(0),
+            context: HashMap::default(),
             context_by_message: HashMap::default(),
             completion_count: 0,
             pending_completions: Vec::new(),
@@ -129,8 +131,15 @@ impl Thread {
         &self.tools
     }
 
-    pub fn context_for_message(&self, id: MessageId) -> Option<&Vec<Context>> {
-        self.context_by_message.get(&id)
+    pub fn context_for_message(&self, id: MessageId) -> Option<Vec<Context>> {
+        let context = self.context_by_message.get(&id)?;
+        Some(
+            context
+                .into_iter()
+                .filter_map(|context_id| self.context.get(&context_id))
+                .cloned()
+                .collect::<Vec<_>>(),
+        )
     }
 
     pub fn pending_tool_uses(&self) -> Vec<&PendingToolUse> {
@@ -144,7 +153,10 @@ impl Thread {
         cx: &mut ModelContext<Self>,
     ) {
         let message_id = self.insert_message(Role::User, text, cx);
-        self.context_by_message.insert(message_id, context);
+        let context_ids = context.iter().map(|context| context.id).collect::<Vec<_>>();
+        self.context
+            .extend(context.into_iter().map(|context| (context.id, context)));
+        self.context_by_message.insert(message_id, context_ids);
     }
 
     pub fn insert_message(