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/main.rs

@@ -13,6 +13,8 @@ mod mcp;
 mod parsers;
 mod sandbox;
 mod supervisor;
+#[cfg(test)]
+mod testing;
 mod vault;
 
 #[macro_use]