Fix panic in overlayfs when saving a file without an overlay (#3849)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Author: Copilot

internal/project/overlayfs.go

@@ -367,12 +367,14 @@ func (fs *overlayFS) processChanges(changes []FileChange) (FileChangeSummary, ma
 		}
 
 		if events.saved {
-			if o == nil {
-				panic("overlay not found for saved file: " + uri)
+			if o != nil {
+				o = newOverlay(o.FileName(), o.Content(), o.Version(), o.kind)
+				o.matchesDiskText = true
+				newOverlays[path] = o
+			} else if !events.watchChanged {
+				// File was saved but never opened via didOpen; treat as a disk change.
+				result.Changed.Add(uri)
 			}
-			o = newOverlay(o.FileName(), o.Content(), o.Version(), o.kind)
-			o.matchesDiskText = true
-			newOverlays[path] = o
 		}
 
 		if events.created && o == nil {

internal/project/overlayfs_test.go

@@ -197,6 +197,23 @@ func TestProcessChanges(t *testing.T) {
 		assert.Assert(t, !fs.getFile(testURI1.FileName()).MatchesDiskText())
 	})
 
+	t.Run("save without overlay should not panic", func(t *testing.T) {
+		t.Parallel()
+		fs := createOverlayFS()
+
+		// Save a file that was never opened (no overlay exists).
+		// This can happen when an editor sends didSave for a file
+		// that is not managed by the LSP server (e.g., package.json).
+		result, _ := fs.processChanges([]FileChange{
+			{
+				Kind: FileChangeKindSave,
+				URI:  testURI1,
+			},
+		})
+		// Should be treated as a disk change
+		assert.Assert(t, result.Changed.Has(testURI1))
+	})
+
 	t.Run("close then open in same batch marks as changed", func(t *testing.T) {
 		t.Parallel()
 		fs := createOverlayFS()