merge: taint-query null-guard fix and trace-enabled engine runner

The query fix stops a class-level call from aborting the whole taint batch on
real PHP code; gate, map_gate and pair_gate outputs are unchanged. Trace parsing
lives with the runner (benchmarks/learn/engine_trace_parse.py).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/openultrasast/model/taint.py

@@ -28,6 +28,7 @@ def request_params(
     call_depth: int = 0,
     hook_callbacks: str = "",
     field_parameter_sources: bool | None = None,
+    trace: bool = False,
 ) -> dict[str, object]:
     """The query parameters this arbiter sends. Shared with the batcher so there is one definition, not two.
 
@@ -40,6 +41,7 @@ def request_params(
     means unscoped, which is what the single-region pair path still uses.
     """
     return {
+        **({"trace": "true"} if trace else {}),
         "sources": spec.sources,
         "sinks": spec.sinks,
         "sanitizers": spec.sanitizers,