test(sdk): give the isolation and retry-negative tests teeth

Three tests that would pass against a broken implementation:

- `kv`/`docs` cross-app isolation asserted only that app B sees nothing — an
  execution_id-keyed (or script_id-keyed) bridge would pass that AND every
  single-execution round-trip, with app-scoped persistence entirely gone. Added
  the positive control: app A re-reads its own value in a LATER execution (a fresh
  execution_id/script_id via `baseline_request`), which only holds if storage is
  keyed by app_id.

- `retry_policy_rejects_bogus_backoff` asserted a bare `is_err()`, which passes if
  `retry::policy` is unregistered (ErrorFunctionNotFound), on a script typo, or if
  every policy is rejected. Now it pins the message (`backoff`) and adds a
  valid-backoff control that must succeed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/executor-core/tests/sdk_kv.rs

@@ -311,6 +311,21 @@ async fn kv_bridge_preserves_cross_app_isolation() {
         let c = kv::collection("shared");
         c.get("k")
     "#;
-    let body = run_script(engine, reader, baseline_request(app_b)).await;
-    assert_eq!(body, Value::Null);
+    let body = run_script(engine.clone(), reader, baseline_request(app_b)).await;
+    assert_eq!(body, Value::Null, "app B must not see app A's value");
+
+    // Positive control — WITHOUT it this test has no teeth. `baseline_request`
+    // mints a fresh execution_id AND script_id each call, so this second run under
+    // app_a is a DIFFERENT execution/script than the writer. A bridge that keyed
+    // storage by execution_id or script_id (instead of app_id) would pass the
+    // negative above AND every single-execution round-trip, while app-scoped
+    // persistence was completely gone. This catches that: app A must still read
+    // back its own value across executions.
+    let body = run_script(engine, reader, baseline_request(app_a)).await;
+    assert_eq!(
+        body,
+        Value::from("from-a"),
+        "app A must read back its own value in a later execution — storage is keyed \
+         by app_id, not execution_id/script_id"
+    );
 }