text_thread: Improve roles after assistant::Split (shift-enter) (#33215)

Default to `You` when triggering `assistant::Split` at the end of a thread

Release Notes:

- agent_thread: Improved roles when triggering `assistant::Split`
(`shift-enter`)

crates/assistant_context/src/assistant_context.rs

@@ -2523,6 +2523,12 @@ impl AssistantContext {
             }
 
             let message = start_message;
+            let at_end = range.end >= message.offset_range.end.saturating_sub(1);
+            let role_after = if range.start == range.end || at_end {
+                Role::User
+            } else {
+                message.role
+            };
             let role = message.role;
             let mut edited_buffer = false;
 
@@ -2557,7 +2563,7 @@ impl AssistantContext {
             };
 
             let suffix_metadata = MessageMetadata {
-                role,
+                role: role_after,
                 status: MessageStatus::Done,
                 timestamp: suffix.id.0,
                 cache: None,