acp: Eagerly load all kinds of mentions (#36741)

This PR makes it so that all kinds of @-mentions start loading their
context as soon as they are confirmed. Previously, we were waiting to
load the context for file, symbol, selection, and rule mentions until
the user's message was sent. By kicking off loading immediately for
these kinds of context, we can support adding selections from unsaved
buffers, and we make the semantics of @-mentions more consistent.

Loading all kinds of context eagerly also makes it possible to simplify
the structure of the MentionSet and the code around it. Now MentionSet
is just a single hash map, all the management of creases happens in a
uniform way in `MessageEditor::confirm_completion`, and the helper
methods for loading different kinds of context are much more focused and
orthogonal.

Release Notes:

- N/A

---------

Co-authored-by: Conrad <conrad@zed.dev>

crates/agent/src/thread_store.rs

@@ -893,8 +893,19 @@ impl ThreadsDatabase {
 
         let needs_migration_from_heed = mdb_path.exists();
 
-        let connection = if *ZED_STATELESS || cfg!(any(feature = "test-support", test)) {
+        let connection = if *ZED_STATELESS {
             Connection::open_memory(Some("THREAD_FALLBACK_DB"))
+        } else if cfg!(any(feature = "test-support", test)) {
+            // rust stores the name of the test on the current thread.
+            // We use this to automatically create a database that will
+            // be shared within the test (for the test_retrieve_old_thread)
+            // but not with concurrent tests.
+            let thread = std::thread::current();
+            let test_name = thread.name();
+            Connection::open_memory(Some(&format!(
+                "THREAD_FALLBACK_{}",
+                test_name.unwrap_or_default()
+            )))
         } else {
             Connection::open_file(&sqlite_path.to_string_lossy())
         };