fix: tolerate sessions without ArrayKernels in executor

`execute_until` and the single-step `Executable::execute` looked up the
`ArrayKernels` session variable with `session.get`, which panics when the
variable is absent. Sessions built without registering `ArrayKernels`
(such as the `run_end_take` benchmark's `VortexSession::empty()`) then
panicked during execution.

Fall back to an empty `ArrayKernels` registry when the variable is not
present, matching the optimizer's `get_opt` contract. The registry is
consulted first and then each child's static `execute_parent` method, so
built-in kernels still fire under an empty registry.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Author: claude

vortex-array/src/executor.rs

@@ -163,7 +163,14 @@ impl ArrayRef {
         let max_iterations = max_iterations();
 
         let session = ctx.session().clone();
-        let kernels = session.get::<ArrayKernels>();
+        let fallback_kernels;
+        let kernels = match session.get_opt::<ArrayKernels>() {
+            Some(kernels) => kernels,
+            None => {
+                fallback_kernels = ArrayKernels::empty();
+                &fallback_kernels
+            }
+        };
 
         for _ in 0..max_iterations {
             let is_done = stack
@@ -427,7 +434,14 @@ impl Executable for ArrayRef {
         }
 
         let session = ctx.session().clone();
-        let kernels = session.get::<ArrayKernels>();
+        let fallback_kernels;
+        let kernels = match session.get_opt::<ArrayKernels>() {
+            Some(kernels) => kernels,
+            None => {
+                fallback_kernels = ArrayKernels::empty();
+                &fallback_kernels
+            }
+        };
 
         if let Some(executed_parent) =
             try_execute_parent(&array, occupied_slots(&array), kernels, ctx)?