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/project/src/task_inventory.rs

@@ -229,10 +229,11 @@ impl Inventory {
     pub fn task_template_by_label(
         &self,
         buffer: Option<Entity<Buffer>>,
+        worktree_id: Option<WorktreeId>,
         label: &str,
         cx: &App,
     ) -> Option<TaskTemplate> {
-        let (worktree_id, file, language) = buffer
+        let (buffer_worktree_id, file, language) = buffer
             .map(|buffer| {
                 let buffer = buffer.read(cx);
                 let file = buffer.file().cloned();
@@ -244,7 +245,7 @@ impl Inventory {
             })
             .unwrap_or((None, None, None));
 
-        self.list_tasks(file, language, worktree_id, cx)
+        self.list_tasks(file, language, worktree_id.or(buffer_worktree_id), cx)
             .iter()
             .find(|(_, template)| template.label == label)
             .map(|val| val.1.clone())