test(state-transition): cover JUSTIFIED_SLOT_OUT_OF_RANGE rejection (leanEthereum#1100)

The defensive guard on the justification-bitfield read path had zero
consensus-vector coverage. A refactor that dropped it, raised a raw
IndexError, or returned not-justified would pass CI and could diverge
clients. Add a state_transition vector that drives a vote whose source
slot sits one past the tracked window and assert the rejection reason
plus the full formatted message.

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

Author: tcoratger

tests/consensus/lstar/state_transition/test_justified_slot_out_of_range.py

@@ -0,0 +1,71 @@
+"""State Transition: justification-bitfield range guard"""
+
+import pytest
+
+from consensus_testing import (
+    AggregatedAttestationSpec,
+    BlockSpec,
+    ExpectedRejection,
+    StateTransitionTestFiller,
+)
+from lean_spec.spec.forks import RejectionReason, Slot, ValidatorIndex
+
+pytestmark = pytest.mark.valid_until("Lstar")
+
+
+def test_source_slot_beyond_tracked_range_rejects_block(
+    state_transition_test: StateTransitionTestFiller,
+) -> None:
+    """
+    A vote whose source slot sits past the tracked justification window rejects the block.
+
+    Given
+    -----
+    - 4 validators.
+    - the chain:
+        genesis -> block_1(1) -> block(2)
+    - finalization stays at genesis, so the tracked window is anchored at slot 0.
+    - processing block(2) tracks justification only for slot 1, a window of length 1.
+    - block(2) carries a forced V0, V1, V2 vote targeting block_1.
+    - the vote's source slot is forced to 2, one past the tracked window.
+
+    When
+    ----
+    - the chain processes block(2).
+
+    Then
+    ----
+    - the source-slot read finds no tracked bit for slot 2.
+    - the block is rejected with JUSTIFIED_SLOT_OUT_OF_RANGE.
+    - the message names slot 2, finalized boundary 0, and tracked length 1.
+    """
+    state_transition_test(
+        blocks=[
+            BlockSpec(slot=Slot(1), label="block_1"),
+            BlockSpec(
+                slot=Slot(2),
+                parent_label="block_1",
+                forced_attestations=[
+                    AggregatedAttestationSpec(
+                        validator_indices=[
+                            ValidatorIndex(0),
+                            ValidatorIndex(1),
+                            ValidatorIndex(2),
+                        ],
+                        slot=Slot(2),
+                        target_slot=Slot(1),
+                        target_root_label="block_1",
+                        source_root_label="block_1",
+                        source_slot=Slot(2),
+                    ),
+                ],
+            ),
+        ],
+        post=None,
+        expected_rejection=ExpectedRejection(
+            reason=RejectionReason.JUSTIFIED_SLOT_OUT_OF_RANGE,
+            exact_message=(
+                "Slot 2 is outside the tracked range (finalized_boundary=0, tracked_length=1)"
+            ),
+        ),
+    )