fix(py): Fix array/list value serialization (#1827)

The rust-side serialization definition for custom constants is a bit
more complex than what we had written on the python side for array and
list values.

This fixes #1826, and was tested by running the issue's example from
guppy.

- > Why aren't we testing this locally?

  Constant value deserialization is a "best effort" thing.
If we can match the serialized thing with one of the hardcoded
implementors of `CustomConst` (registered in a global var by `typetag`)
then we deserialize into the specific class.
If we cannot deserialize into any specific class, we use
`CustomSerialized` by default.
  
So even if we build a constant from python and `validate` using the rust
checker, the deserialization will succeed and validate correctly.
  
It'd be nice to fail the deserialization if a constant claims to be of
some specific kind but cannot be deserialized into it. I'll open an
issue and look a bit into it.

hugr-py/src/hugr/std/collections/array.py

@@ -60,7 +60,7 @@ class ArrayVal(val.ExtensionValue):
     """Constant value for a statically sized array of elements."""
 
     v: list[val.Value]
-    ty: tys.Type
+    ty: Array
 
     def __init__(self, v: list[val.Value], elem_ty: tys.Type) -> None:
         self.v = v
@@ -71,7 +71,11 @@ def to_value(self) -> val.Extension:
         # The value list must be serialized at this point, otherwise the
         # `Extension` value would not be serializable.
         vs = [v._to_serial_root() for v in self.v]
-        return val.Extension(name, typ=self.ty, val=vs, extensions=[EXTENSION.name])
+        element_ty = self.ty.ty._to_serial_root()
+        serial_val = {"values": vs, "typ": element_ty}
+        return val.Extension(
+            name, typ=self.ty, val=serial_val, extensions=[EXTENSION.name]
+        )
 
     def __str__(self) -> str:
         return f"array({comma_sep_str(self.v)})"

hugr-py/src/hugr/std/collections/list.py

@@ -39,7 +39,7 @@ class ListVal(val.ExtensionValue):
     """Constant value for a list of elements."""
 
     v: list[val.Value]
-    ty: tys.Type
+    ty: List
 
     def __init__(self, v: list[val.Value], elem_ty: tys.Type) -> None:
         self.v = v
@@ -50,7 +50,11 @@ def to_value(self) -> val.Extension:
         # The value list must be serialized at this point, otherwise the
         # `Extension` value would not be serializable.
         vs = [v._to_serial_root() for v in self.v]
-        return val.Extension(name, typ=self.ty, val=vs, extensions=[EXTENSION.name])
+        element_ty = self.ty.ty._to_serial_root()
+        serial_val = {"values": vs, "typ": element_ty}
+        return val.Extension(
+            name, typ=self.ty, val=serial_val, extensions=[EXTENSION.name]
+        )
 
     def __str__(self) -> str:
         return f"[{comma_sep_str(self.v)}]"