fix(gfql): gate DAG NIE pass-through to polars; skip polars conformance without polars

Two CI failures on this layer, both diagnosed from the failing 3.8/3.14 jobs:

(A, real regression) execute_call() added an unconditional
'if isinstance(error, NotImplementedError): raise error' to propagate the polars
no-silent-bridge decline on the DAG surface — but it ALSO intercepted a
pandas/cudf NIE like fa2_layout's 'requires a GPU', so it stopped falling through
to the GFQLTypeError(E303) wrapper and test_fa2_layout_cpu_requires_gpu failed.
Gate the pass-through to engine in (POLARS, POLARS_GPU); pandas/cudf NIEs wrap to
E303 as before. Verified: fa2 test passes, full test_call_operations green (24/2).

(B, py3.14 env) test_engine_polars_conformance_matrix hardcodes the polars lane
without probing importability; on Python 3.14 (no cp314 polars wheel) polars is
absent from the lockfile, so every case reported a non-NIE ImportError as a
conformance failure (~60). Add module-level pytest.importorskip('polars') so the
matrix skips cleanly when polars is unavailable.

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

Author: lmeyerov

graphistry/compute/gfql/call/executor.py

@@ -253,9 +253,11 @@ def execute_call(g: Plottable, function: str, params: Dict[str, Any], engine: En
         ) from error
     if isinstance(error, GFQLTypeError):
         raise error
-    if isinstance(error, NotImplementedError):
+    if isinstance(error, NotImplementedError) and engine in (Engine.POLARS, Engine.POLARS_GPU):
         # Honest engine-capability decline (e.g. the polars no-silent-bridge guard above) —
         # propagate as-is so the DAG surface matches the chain surface's NotImplementedError.
+        # Gated to the polars engines: a pandas/cudf NIE (e.g. fa2_layout requiring a GPU) must
+        # still fall through to the GFQLTypeError(E303) wrapper below, not leak as a bare NIE.
         raise error
     if error is not None:
         raise GFQLTypeError(

graphistry/tests/compute/gfql/test_engine_polars_conformance_matrix.py

@@ -15,6 +15,11 @@
 import graphistry
 from graphistry.compute.ast import n, e_forward, e_reverse, e_undirected, call, let, rows, with_, return_
 
+# The whole matrix is the CPU polars lane (plus a GPU lane when present) — it requires polars.
+# On interpreters without a polars wheel (e.g. Python 3.14, which has no cp314 wheel yet), skip
+# the module cleanly rather than reporting every case as a non-NIE ImportError "conformance" fail.
+pytest.importorskip("polars")
+
 
 # ---- graph with diverse dtypes (int, float w/ null, str, bool) ----
 def _graph(seed=0):