Improve distinguishing user from agent edits (#34716)

We no longer rely on the `author` field to tell if a change was made by
the user or the agent. The `author` can be set to `User` in many
situations that are not really user-made edits, such as saving a file,
accepting a change, auto-formatting, and more. I started tracking and
fixing some of these cases, but found that inspecting changes in
`diff_base` is a more reliable method.

Also, we no longer show empty diffs. For example, if the user adds a
line and then removes the same line, the final diff is empty, even
though the buffer is marked as user-changed. Now we won't show such
edit.

There are still some issues to address:

- When a user edits within an unaccepted agent-written block, this
change becomes a part of the agent's edit. Rejecting this block will
lose user edits. It won't be displayed in project notifications, either.

- Accepting an agent block counts as a user-made edit.

- Agent start to call `project_notifications` tool after seeing enough
auto-calls.

Release Notes:

- N/A

crates/assistant_tools/src/edit_file_tool.rs

@@ -278,6 +278,9 @@ impl Tool for EditFileTool {
                 .unwrap_or(false);
 
             if format_on_save_enabled {
+                action_log.update(cx, |log, cx| {
+                    log.buffer_edited(buffer.clone(), cx);
+                })?;
                 let format_task = project.update(cx, |project, cx| {
                     project.format(
                         HashSet::from_iter([buffer.clone()]),