assistant2: Add testing environment variables (#27789)

To make it easier to design UIs for some of these scenarios. This PR
adds specifically two variables:
- `ZED_SIMULATE_NO_THREAD_HISTORY`
- `ZED_SIMULATE_NO_LLM_PROVIDER`

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/assistant2/src/history_store.rs

@@ -43,6 +43,11 @@ impl HistoryStore {
     pub fn entries(&self, cx: &mut Context<Self>) -> Vec<HistoryEntry> {
         let mut history_entries = Vec::new();
 
+        #[cfg(debug_assertions)]
+        if std::env::var("ZED_SIMULATE_NO_THREAD_HISTORY").is_ok() {
+            return history_entries;
+        }
+
         for thread in self.thread_store.update(cx, |this, _cx| this.threads()) {
             history_entries.push(HistoryEntry::Thread(thread));
         }

crates/language_model/src/registry.rs

@@ -203,6 +203,11 @@ impl LanguageModelRegistry {
     }
 
     pub fn active_provider(&self) -> Option<Arc<dyn LanguageModelProvider>> {
+        #[cfg(debug_assertions)]
+        if std::env::var("ZED_SIMULATE_NO_LLM_PROVIDER").is_ok() {
+            return None;
+        }
+
         Some(self.active_model.as_ref()?.provider.clone())
     }