test(web): lock finalize SessionNotFinishedError divergence + docstring note (Task 1 follow-up)

proteus/runtime/_session_core.py

@@ -4,7 +4,10 @@ The per-session logic (build + Cut replay, one-turn TurnTrace construction,
 finalize -> scored SessionTrace) lives here so both SessionRunner (agent-driven,
 batch) and InteractiveSession (HTTP-driven, stepwise) produce *identical* traces
 from the same actions. This is a pure extraction of what used to be inline in
-SessionRunner -- no behavior change (pinned by the existing suite).
+SessionRunner -- no behavior change (pinned by the existing suite). The one
+intentional deviation: ``finalize`` raises ``SessionNotFinishedError`` where the
+original used a bare ``assert`` (which vanishes under ``python -O``); the
+terminal condition checked is identical.
 """
 from __future__ import annotations
 

tests/runtime/test_session_core.py

@@ -2,6 +2,8 @@
 and InteractiveSession)."""
 from __future__ import annotations
 
+import pytest
+
 import proteus.grid  # noqa: F401  (registers predator_evade)
 from proteus.grid.difficulty import Difficulty
 from proteus.runtime import _session_core as core
@@ -59,3 +61,17 @@ def test_finalize_produces_scored_trace():
         "motive_reading_accuracy", "survival_fraction", "away_move_fraction",
         "trajectory_agreement", "final_distance_gap",
     }
+
+
+def test_finalize_before_terminal_or_budget_raises():
+    # The sole intentional divergence from the original SessionRunner: a
+    # non-terminal, under-budget finalize raises (vs the old bare assert).
+    built = core.build_session("predator_evade", 42, Difficulty.EASY, 5)
+    assert not (built.game.eliminated or built.game.survived)
+    with pytest.raises(core.SessionNotFinishedError):
+        core.finalize(
+            "predator_evade", built.scenario, built.game,
+            seed=42, difficulty=Difficulty.EASY, play_turns=5,
+            turns=[], cut_frames=built.cut_frames,
+            motive_category="survival", model="human",
+        )