feat(slots-rpc): available_slots capacity hint 추가 (BAT-037 / REQ-073 AC-6)

slots RPC 응답에 available_slots: int|null 포함.
max_active_slots 미설정 시 null, 설정 시 max(0, M-active).
16 tests passed (AC-6 신규 3개 포함), 423 전체 통과.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/application/test_slots_handler.py

@@ -32,6 +32,7 @@ def _wire(items=None, *, max_active_slots=None):
         snapshot_store=store,
         pause_store=pause,
         watchdog_store=watchdog,
+        max_active_slots=max_active_slots,
     )
     return repo, svc, store, pause, watchdog, handler
 
@@ -45,13 +46,12 @@ def _req(filter=None, spy=False):
     return RpcRequest(payload=payload, correlation_id="test-corr")
 
 
-# REQ-073 AC-3: response keys only {slots/degraded/reason/message}
+# REQ-073 AC-3: response keys include available_slots; assets/watchdog/strategy absent
 def test_response_keys_only_slots_health():
     _, _, _, _, _, handler = _wire()
     resp = handler(_req())
-    assert set(resp.payload.keys()) == {"slots", "degraded", "reason", "message"}
+    assert set(resp.payload.keys()) == {"slots", "available_slots", "degraded", "reason", "message"}
     assert "assets" not in resp.payload
-    assert "available_slots" not in resp.payload
     assert "watchdog" not in resp.payload
 
 
@@ -210,3 +210,34 @@ def test_spy_false_updates_watchdog():
     sid = str(holding.slot_id)
     handler(_req(filter="holding", spy=False))
     assert watchdog.last_seen(sid) is not None  # updated
+
+
+# REQ-073 AC-6: available_slots capacity hint
+def test_available_slots_null_when_uncapped():
+    _, _, _, _, _, handler = _wire(max_active_slots=None)
+    resp = handler(_req())
+    assert resp.payload["available_slots"] is None
+
+
+def test_available_slots_remaining_capacity():
+    repo, svc, _, _, _, handler = _wire(max_active_slots=2)
+    # 0 active → available = 2
+    assert handler(_req()).payload["available_slots"] == 2
+    # buy pf-A into holding → 1 active, available = 1
+    slot_a = repo.find_slot_by_portfolio("default", "pf-A")
+    pending = svc.transit_slot(slot_a.slot_id, SlotState.INACTIVE, SlotState.PENDING_BUY,
+                               reason="buy", actor="test", symbol="005930")
+    svc.transit_slot(pending.slot_id, SlotState.PENDING_BUY, SlotState.HOLDING,
+                     reason="filled", actor="test", avg_price=100.0, qty=5.0)
+    assert handler(_req()).payload["available_slots"] == 1
+
+
+def test_available_slots_zero_when_full():
+    repo, svc, _, _, _, handler = _wire(max_active_slots=1)
+    slot_a = repo.find_slot_by_portfolio("default", "pf-A")
+    pending = svc.transit_slot(slot_a.slot_id, SlotState.INACTIVE, SlotState.PENDING_BUY,
+                               reason="buy", actor="test", symbol="005930")
+    svc.transit_slot(pending.slot_id, SlotState.PENDING_BUY, SlotState.HOLDING,
+                     reason="filled", actor="test", avg_price=100.0, qty=5.0)
+    # M=1, active=1 → available=0
+    assert handler(_req()).payload["available_slots"] == 0