debugger: Fix spec violation with threads request being issued before debug session is initialized (#33627)

Follow-up to #32852. This time we'll check if the debug session is
initialized before querying threads.

Release Notes:

- Fix Zed's debugger issuing threads request before it is allowed to do
so per DAP specification.

crates/project/src/debugger/session.rs

@@ -1037,10 +1037,6 @@ impl Session {
         matches!(self.mode, Mode::Building)
     }
 
-    pub fn is_running(&self) -> bool {
-        matches!(self.mode, Mode::Running(_))
-    }
-
     pub fn as_running_mut(&mut self) -> Option<&mut RunningMode> {
         match &mut self.mode {
             Mode::Running(local_mode) => Some(local_mode),