unifying all image saves to /tmp to bug-proof (#14149)

image-gen feature will have the model saving to /tmp by default + at all
times

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__image_generation_call_history_snapshot.snap

@@ -5,4 +5,4 @@ expression: combined
 ---
 • Generated Image:
   └ A tiny blue square
-  └ Saved to: /tmp/project
+  └ Saved to: /tmp

codex-rs/tui/src/chatwidget/tests.rs

@@ -6289,7 +6289,7 @@ async fn image_generation_call_adds_history_cell() {
             status: "completed".into(),
             revised_prompt: Some("A tiny blue square".into()),
             result: "Zm9v".into(),
-            saved_path: Some("/tmp/project/ig-1.png".into()),
+            saved_path: Some("/tmp/ig-1.png".into()),
         }),
     });