p5a: fix a deadlock in the given TestFetchIf (lock held across a request); run notes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/p5a/_files/internal/fetch/pdf_test.go

@@ -167,9 +167,10 @@ func TestFetchIf(t *testing.T) {
 		t.Fatalf("changed document: res %+v err %v; want 200 with the new body and ETag", res, err)
 	}
 	mu.Lock()
-	defer mu.Unlock()
-	if len(got) != 2 || got[0] != `"v1"|` || got[1] != `"old"|Thu, 24 Sep 2026 12:00:00 GMT` {
-		t.Errorf("validators sent: %q", got)
+	sent := append([]string(nil), got...)
+	mu.Unlock() // never hold mu across another request: the handler takes it
+	if len(sent) != 2 || sent[0] != `"v1"|` || sent[1] != `"old"|Thu, 24 Sep 2026 12:00:00 GMT` {
+		t.Errorf("validators sent: %q", sent)
 	}
 	// With no validators FetchIf is a plain Fetch.
 	if _, err := f.FetchIf(context.Background(), ts.URL, "", ""); err != nil {