fix(testing): stop dropping an explicit proposer index of 0 (#1157)

resolve_proposer_index used `self.proposer_index or <default>`, but ValidatorIndex
subclasses int with no __bool__, so ValidatorIndex(0) is falsy and a proposer pinned to
validator 0 was silently replaced by the round-robin default. Use an explicit None check.
Vectors that never pin proposer 0 are unchanged.

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

Author: tcoratger

packages/testing/src/consensus_testing/test_types/block_spec.py

@@ -73,9 +73,9 @@ class BlockSpec(CamelModel):
 
     def resolve_proposer_index(self, num_validators: int) -> ValidatorIndex:
         """Return the proposer index, falling back to the spec's round-robin schedule."""
-        return self.proposer_index or ValidatorIndex.proposer_for_slot(
-            self.slot, Uint64(num_validators)
-        )
+        if self.proposer_index is not None:
+            return self.proposer_index
+        return ValidatorIndex.proposer_for_slot(self.slot, Uint64(num_validators))
 
     def resolve_parent_root(
         self,