Scope slash commands, context servers, and tools to individual Assistant Panel instances (#20372)

This PR reworks how the Assistant Panel references slash commands,
context servers, and tools.

Previously we were always reading them from the global registries, but
now we store individual collections on each Assistant Panel instance so
that there can be different ones registered for each project.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Joseph <joseph@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/collab/src/tests/integration_tests.rs

@@ -6,7 +6,7 @@ use crate::{
     },
 };
 use anyhow::{anyhow, Result};
-use assistant::{ContextStore, PromptBuilder};
+use assistant::{ContextStore, PromptBuilder, SlashCommandWorkingSet, ToolWorkingSet};
 use call::{room, ActiveCall, ParticipantLocation, Room};
 use client::{User, RECEIVE_TIMEOUT};
 use collections::{HashMap, HashSet};
@@ -6489,11 +6489,27 @@ async fn test_context_collaboration_with_reconnect(
 
     let prompt_builder = Arc::new(PromptBuilder::new(None).unwrap());
     let context_store_a = cx_a
-        .update(|cx| ContextStore::new(project_a.clone(), prompt_builder.clone(), cx))
+        .update(|cx| {
+            ContextStore::new(
+                project_a.clone(),
+                prompt_builder.clone(),
+                Arc::new(SlashCommandWorkingSet::default()),
+                Arc::new(ToolWorkingSet::default()),
+                cx,
+            )
+        })
         .await
         .unwrap();
     let context_store_b = cx_b
-        .update(|cx| ContextStore::new(project_b.clone(), prompt_builder.clone(), cx))
+        .update(|cx| {
+            ContextStore::new(
+                project_b.clone(),
+                prompt_builder.clone(),
+                Arc::new(SlashCommandWorkingSet::default()),
+                Arc::new(ToolWorkingSet::default()),
+                cx,
+            )
+        })
         .await
         .unwrap();