feat(backend): c2 callbacks + execute endpoints (sprint 8 M2)

- Add C2Error exception to adapter ABC
- Add promote_to_in_progress() helper to simulation_workflow (pending→in_progress)
- Flesh out MythicAdapter: list_callbacks() (GraphQL query) + create_task() (mutation)
- Expand FakeAdapter to 3 deterministic callbacks; switch task store to per-instance
- Add GET /api/engagements/<id>/c2/callbacks — lists active callbacks via adapter
- Add POST /api/simulations/<id>/c2/execute — issues tasks, stores C2Task rows,
  auto-transitions pending→in_progress, blocks on done (409)
- Both endpoints: SOC=403, 503 no-key, 502 adapter error, sanitized error messages
- Add requests-mock==1.12.1 to requirements.txt
- 42 new tests (342 total, 300 M1 baseline preserved green)

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

backend/tests/test_c2_adapter_fake_m2.py

@@ -0,0 +1,62 @@
+"""FakeAdapter M2 tests — list_callbacks shape, create_task monotonicity."""
+from __future__ import annotations
+
+from backend.app.services.c2.fake import FakeAdapter
+
+
+class TestFakeAdapterListCallbacks:
+    def test_returns_three_callbacks(self):
+        adapter = FakeAdapter()
+        callbacks = adapter.list_callbacks()
+        assert len(callbacks) == 3
+
+    def test_all_active(self):
+        adapter = FakeAdapter()
+        for cb in adapter.list_callbacks():
+            assert cb.active is True
+
+    def test_display_ids_are_1_2_3(self):
+        adapter = FakeAdapter()
+        ids = [cb.display_id for cb in adapter.list_callbacks()]
+        assert ids == [1, 2, 3]
+
+    def test_pinned_last_checkin_format(self):
+        adapter = FakeAdapter()
+        for cb in adapter.list_callbacks():
+            assert cb.last_checkin.startswith("2026-06-10")
+
+    def test_callbacks_have_host_user_domain(self):
+        adapter = FakeAdapter()
+        for cb in adapter.list_callbacks():
+            assert cb.host
+            assert cb.user
+            assert cb.domain
+
+
+class TestFakeAdapterCreateTask:
+    def test_returns_monotonic_ids_from_1000(self):
+        adapter = FakeAdapter()
+        id1 = adapter.create_task(1, "whoami")
+        id2 = adapter.create_task(1, "ipconfig")
+        assert id1 == 1000
+        assert id2 == 1001
+
+    def test_separate_instances_start_at_1000_independently(self):
+        a1 = FakeAdapter()
+        a2 = FakeAdapter()
+        assert a1.create_task(1, "cmd") == 1000
+        assert a2.create_task(1, "cmd") == 1000
+
+    def test_stores_command_and_callback(self):
+        adapter = FakeAdapter()
+        tid = adapter.create_task(callback_display_id=2, command="ls", params="-la")
+        task = adapter._tasks[tid]
+        assert task["command"] == "ls"
+        assert task["params"] == "-la"
+        assert task["callback_display_id"] == 2
+
+    def test_initial_status_submitted(self):
+        adapter = FakeAdapter()
+        tid = adapter.create_task(1, "hostname")
+        assert adapter._tasks[tid]["status"] == "submitted"
+        assert adapter._tasks[tid]["completed"] is False