boundary spec hint the validation type

tidy3d/components/boundary.py

@@ -66,7 +66,7 @@ def bloch_phase(self) -> Complex:
 
     @classmethod
     def from_source(
-        cls, source: BlochSourceType, domain_size: float, axis: Axis, medium: Medium = None
+            cls, source: BlochSourceType, domain_size: float, axis: Axis, medium: Medium = None
     ) -> BlochBoundary:
         """Set the Bloch vector component based on a given angled source and its center frequency.
            Note that if a broadband angled source is used, only the frequency components near the
@@ -188,7 +188,7 @@ class PMLParams(AbsorberParams):
         3,
         title="Kappa Order",
         description="Order of the polynomial describing the PML kappa profile "
-        "(kappa~dist^kappa_order).",
+                    "(kappa~dist^kappa_order).",
     )
 
     kappa_min: pd.NonNegativeFloat = pd.Field(0.0, title="Kappa Minimum", description="")
@@ -343,12 +343,14 @@ class Boundary(Tidy3dBaseModel):
 
     plus: BoundaryEdgeType = pd.Field(
         Periodic(),
+        discriminator="type",
         title="Plus BC",
         description="Boundary condition on the plus side along a dimension.",
     )
 
     minus: BoundaryEdgeType = pd.Field(
         Periodic(),
+        discriminator="type",
         title="Minus BC",
         description="Boundary condition on the minus side along a dimension.",
     )