test(discovery): end-to-end errand_runner session emits discovery metric

tests/runtime/test_errand_discovery.py

@@ -82,3 +82,36 @@ def test_interactive_accepts_interact_for_errand():
     s = InteractiveSession("errand_runner", seed=7, play_turns=5, use_default_memory=False)
     assert "interact" in s.state()["actions"]
     s.step("interact")  # must not raise "invalid action"
+
+
+def test_full_session_runs_and_emits_discovery_metric():
+    """End-to-end SessionRunner with a fake provider that always reports the
+    correct body index; confirms discovery_turn and discovery_identified are
+    emitted on the resulting SessionTrace.metrics."""
+    from proteus.providers.fake import FakeProvider  # deterministic, no network
+    from proteus.game.agents.vanilla import VanillaAgent
+    from proteus.game.runtime.session import SessionRunner
+
+    # Confirm the true_body_index for seed 7 (standalone and runner use same RNG).
+    scen = get_scenario("errand_runner")()
+    MotiveGridGame(scen, random.Random(7), Difficulty.EASY, max_steps=10)
+    true = scen.true_body_index  # == 1 for seed 7
+
+    # FakeProvider.responses repeats last entry once exhausted, so a single
+    # response is sufficient for any play_turns count.
+    provider = FakeProvider(responses=[f"SELF: {true}\nACTION: stay"])
+    agent = VanillaAgent(provider)
+    runner = SessionRunner(
+        "errand_runner", agent, seed=7, play_turns=4,
+        use_probe=False, motive_category="errand",
+    )
+    trace = runner.run()
+
+    assert trace.scenario == "errand_runner"
+    assert "discovery_turn" in trace.metrics, (
+        f"discovery_turn missing from metrics: {trace.metrics}"
+    )
+    # Every turn reported the correct body index -> identified at turn 1.
+    assert trace.metrics["discovery_identified"] == 100.0, (
+        f"Expected 100.0 but got {trace.metrics['discovery_identified']}"
+    )