debugger: Fix debug scenario's defined in debug.json not using passed in build task (#29973)

There were two bugs that caused user-defined debug scenarios from being
able to run a build task.

1. DebugRequest would be deserialized to `Attach` even when `process_id`
wasn't defined in a user's configuration file. This has been fixed by
adding our own deserializer that defaults to None if there are no fields
present instead of `Attach`, and I added tests to prevent regressions.
2. Debug scenario resolve phase never got the active buffer when
spawning a debug session from the new session modal. This has been
worked around by passing in the worktree_id of a debug scenario in the
scenario picker and the active worktree_id otherwise.

Release Notes:

- N/A

crates/debugger_ui/src/attach_modal.rs

@@ -237,7 +237,7 @@ impl PickerDelegate for AttachModalDelegate {
             .flatten();
         if let Some(panel) = panel {
             panel.update(cx, |panel, cx| {
-                panel.start_session(scenario, Default::default(), None, window, cx);
+                panel.start_session(scenario, Default::default(), None, None, window, cx);
             });
         }