Make LinkabelElementSet non-optional in FilterSpecResolution

The LinkableElementSet object is always present in the extant
initializer calls for FilterSpecResolution. This change allows
us to acces it without having to handle the Optional type everywhere,
and prevents future drift into mayhem as we inspect this object
for things like predicate pushdown evaluation.

metricflow-semantics/metricflow_semantics/query/group_by_item/filter_spec_resolution/filter_spec_lookup.py

@@ -171,29 +171,40 @@ class FilterSpecResolution:
 
     lookup_key: ResolvedSpecLookUpKey
     where_filter_intersection: WhereFilterIntersection
-    resolved_linkable_element_set: Optional[LinkableElementSet]
+    resolved_linkable_element_set: LinkableElementSet
     spec_pattern: SpecPattern
     issue_set: MetricFlowQueryResolutionIssueSet
     # Used for error messages.
     filter_location_path: MetricFlowQueryResolutionPath
     object_builder_str: str
 
-    def __post_init__(self) -> None:  # noqa: D105
-        if self.resolved_linkable_element_set is not None:
-            assert len(self.resolved_linkable_element_set.specs) <= 1
+    def __post_init__(self) -> None:
+        """Validation to ensure there is exactly one resolved spec for a FilterSpecResolution.
+
+        Due to the way the FilterSpecResolution is structured, the final output should contain a single spec.
+        """
+        num_specs = len(self.resolved_linkable_element_set.specs)
+        assert num_specs <= 1, (
+            f"Found {num_specs} in {self.resolved_linkable_element_set}, but a valid FilterSpecResolution should "
+            "contain either 0 or 1 resolved specs."
+        )
 
     @property
-    def resolved_spec(self) -> Optional[LinkableInstanceSpec]:  # noqa: D102
-        if self.resolved_linkable_element_set is None:
-            return None
+    def resolved_spec(self) -> Optional[LinkableInstanceSpec]:
+        """Returns the lone resolved spec, if one was found.
 
+        The final ValueError should not be reachable due to the post-init validation, but is in place in case someone
+        updates or removes the latter without accounting for the possibility of runtime divergence.
+        """
         specs = self.resolved_linkable_element_set.specs
         if len(specs) == 0:
             return None
         elif len(specs) == 1:
             return specs[0]
         else:
-            raise RuntimeError(f"Found {len(specs)} in {self.resolved_linkable_element_set}")
+            raise ValueError(
+                f"Found {len(specs)} in {self.resolved_linkable_element_set}, this should not be possible!"
+            )
 
 
 CallParameterSet = Union[