feat(TASK-016): engine + test fixes from the Phase-1 re-review

- warn-per-retry test via shared test-only WarnCollector (src/testing.rs)
- extraction-placement test: extraction failures are never retried
- is_transient_error: typed reqwest/Elapsed pre-check + anchored 429 matchers
- outcome_from docstring corrected; validator warning for
  fallback+output_schema without state_updates (llm + agent)
- validator error for max_attempts == 0 (llm + agent); runtime backstop kept
- nits: generalized branch-escape error text, Continue payload comment,
  BFS test assignments assert, node_id threaded into AgentNodeExecutor warns
- bundled-graph warning baseline records 3 true-positive step-runner nodes
  (orient, edge_case_sweep, write_handoff) pending state_updates there

src/testing.rs

@@ -0,0 +1,43 @@
+//! Test-only helpers shared across modules' unit tests.
+
+use log::{Level, LevelFilter, Log, Metadata, Record};
+use std::sync::{Mutex, Once, OnceLock};
+
+struct WarnCollector;
+
+static WARN_MESSAGES: OnceLock<Mutex<Vec<String>>> = OnceLock::new();
+
+/// Every warn-level message captured since the collector was installed. The
+/// buffer is process-global and shared by all tests, so assert by filtering
+/// for a marker unique to the test rather than on the whole buffer.
+pub(crate) fn warn_messages() -> &'static Mutex<Vec<String>> {
+    WARN_MESSAGES.get_or_init(Mutex::default)
+}
+
+impl Log for WarnCollector {
+    fn enabled(&self, metadata: &Metadata) -> bool {
+        metadata.level() <= Level::Warn
+    }
+
+    fn log(&self, record: &Record) {
+        if self.enabled(record.metadata()) {
+            warn_messages()
+                .lock()
+                .unwrap()
+                .push(record.args().to_string());
+        }
+    }
+
+    fn flush(&self) {}
+}
+
+/// Installs the process-wide warn collector. `log::set_logger` accepts one
+/// logger per process, so every test that captures warns must install through
+/// this shared entry point.
+pub(crate) fn install_warn_collector() {
+    static INSTALL: Once = Once::new();
+    INSTALL.call_once(|| {
+        log::set_logger(&WarnCollector).expect("no other logger should be installed");
+        log::set_max_level(LevelFilter::Warn);
+    });
+}