docs(testing): use bullets for the aggregation_bits field docstring

Per the doc-writer rules, render the field's three behaviors as bullet
points instead of prose, and fix the summary line so it describes the
default (derived) case rather than only the verbatim override.

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

Author: tcoratger

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

@@ -161,11 +161,11 @@ class AggregatedAttestationSpec(AttestationSpec):
 
     aggregation_bits: AggregationBits | None = None
     """
-    Raw aggregation bits placed into the block body verbatim.
+    Raw aggregation bits for the block body, overriding index derivation.
 
-    When unset, bits are derived from the validator indices.
-    Only the state transition forced-attestation path honors an override.
-    Signed paths always derive bits from the validator indices.
+    - When unset, bits are derived from the validator indices.
+    - When set, the bits are used verbatim, even when zero-length or padded.
+    - Only the unsigned forced-attestation path honors the override.
     """
 
     def resolve_aggregation_bits(self) -> AggregationBits: