Refactor cleanupNotebook for clarity and fix keep-ipynb edge case

Restructure cleanupNotebook with early returns that separate three
concerns: non-transient notebooks, keep-ipynb preservation, and
transient deletion.

Previously, when keep-ipynb: true was set but the fileInformationCache
missed on target.source (e.g., path normalization mismatches or
preview re-render invalidation), the else-if branch would still
delete the file. The refactored logic checks keep-ipynb first and
short-circuits before the cache lookup, so the user's explicit
keep-ipynb: true is always honored regardless of cache state.

Cache update remains an internal optimization for later project-level
cleanup — not the authoritative signal for whether to delete.

Author: cderv

src/execute/jupyter/jupyter.ts

@@ -863,16 +863,26 @@ function cleanupNotebook(
   format: Format,
   project: EngineProjectContext,
 ) {
-  // Make notebook non-transient when keep-ipynb is set
   const data = target.data as JupyterTargetData;
-  const cached = project.fileInformationCache.get(target.source);
-  if (cached && data.transient && format.execute[kKeepIpynb]) {
-    if (cached.target && cached.target.data) {
+
+  // Nothing to do for non-transient notebooks (user-authored .ipynb files)
+  if (!data.transient) {
+    return;
+  }
+
+  // User wants to keep the notebook — do not delete.
+  // Also mark the cache entry (if any) as non-transient so later cache
+  // cleanup doesn't remove it.
+  if (format.execute[kKeepIpynb]) {
+    const cached = project.fileInformationCache.get(target.source);
+    if (cached && cached.target && cached.target.data) {
       (cached.target.data as JupyterTargetData).transient = false;
     }
-  } else if (data.transient) {
-    safeRemoveSync(target.input);
+    return;
   }
+
+  // Otherwise the transient notebook is no longer needed; delete it.
+  safeRemoveSync(target.input);
 }
 
 interface JupyterTargetData {