feat(backend): c2 callback history + task import (sprint 8 M4)

Command source decision: extended C2TaskStatus with command: str | None
(default None). Added command_name to _GET_TASK_QUERY so get_task() returns
command in a single round-trip — no separate history fetch needed on import.
4-line change, zero cascading test impact.

adapter.py:
- C2TaskStatus: add command: str | None = None field
- C2HistoricalTask: new dataclass (display_id, command, params, status,
  completed, timestamp) for history rows
- C2TaskPage.items: typed as list[C2HistoricalTask] (was list[dict])

mythic.py:
- _GET_TASK_QUERY: add command_name field
- _LIST_CALLBACK_TASKS_QUERY: new query (order_by id desc, limit/offset)
- _COUNT_CALLBACK_TASKS_QUERY: new aggregate query for total
- get_task(): surfaces command_name as status.command
- list_callback_tasks(): two _post() calls (tasks + count), allow_redirects=False

fake.py:
- _FAKE_HISTORY: frozen deterministic history (cb1=12, cb2=0, cb3=5 tasks)
- list_callback_tasks(): serves from _FAKE_HISTORY, pagination applied
- get_task(): returns command from _tasks dict

api/c2.py:
- GET /api/engagements/<eid>/c2/callbacks/<cid>/history: page+page_size
  defaults 1/25, cap 100, reject <1, 502 on adapter error
- POST /api/simulations/<sid>/c2/import: idempotent per (sim,mythic_id) pair,
  source=import, completed tasks get output+mapping_applied, incomplete tasks
  stored for poll-on-read pickup, auto-transition pending→in_progress

60 new tests (456 total); pytest/ruff/mypy all green

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

backend/tests/test_c2_adapter_fake_m4.py

@@ -0,0 +1,75 @@
+"""FakeAdapter M4 tests — list_callback_tasks pagination."""
+from __future__ import annotations
+
+import pytest
+
+from backend.app.services.c2.adapter import C2HistoricalTask
+from backend.app.services.c2.fake import FakeAdapter
+
+
+@pytest.fixture()
+def adapter() -> FakeAdapter:
+    return FakeAdapter()
+
+
+class TestFakeAdapterListCallbackTasks:
+    def test_callback_1_returns_12_total(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=1, page=1, page_size=25)
+        assert page.total == 12
+
+    def test_callback_2_returns_0_tasks(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=2, page=1, page_size=25)
+        assert page.total == 0
+        assert page.items == []
+
+    def test_callback_3_returns_5_tasks(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=3, page=1, page_size=25)
+        assert page.total == 5
+        assert len(page.items) == 5
+
+    def test_items_are_c2_historical_task_instances(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=1, page=1, page_size=5)
+        for item in page.items:
+            assert isinstance(item, C2HistoricalTask)
+
+    def test_pagination_page1(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=1, page=1, page_size=5)
+        assert len(page.items) == 5
+        assert page.page == 1
+        assert page.page_size == 5
+
+    def test_pagination_page2(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=1, page=2, page_size=5)
+        assert len(page.items) == 5
+        assert page.page == 2
+
+    def test_pagination_last_page_partial(self, adapter):
+        # 12 tasks, page_size=5 → page 3 has 2 items.
+        page = adapter.list_callback_tasks(callback_display_id=1, page=3, page_size=5)
+        assert len(page.items) == 2
+        assert page.total == 12
+
+    def test_pagination_beyond_range_returns_empty(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=1, page=99, page_size=25)
+        assert len(page.items) == 0
+        assert page.total == 12
+
+    def test_history_is_deterministic_across_instances(self):
+        a1 = FakeAdapter()
+        a2 = FakeAdapter()
+        p1 = a1.list_callback_tasks(callback_display_id=1, page=1, page_size=25)
+        p2 = a2.list_callback_tasks(callback_display_id=1, page=1, page_size=25)
+        assert [t.display_id for t in p1.items] == [t.display_id for t in p2.items]
+
+    def test_completed_and_submitted_mix(self, adapter):
+        """Callback 1 has alternating completed/submitted tasks (even=completed)."""
+        page = adapter.list_callback_tasks(callback_display_id=1, page=1, page_size=12)
+        completed = [t for t in page.items if t.completed]
+        submitted = [t for t in page.items if not t.completed]
+        assert len(completed) == 6
+        assert len(submitted) == 6
+
+    def test_unknown_callback_returns_empty(self, adapter):
+        page = adapter.list_callback_tasks(callback_display_id=999, page=1, page_size=25)
+        assert page.total == 0
+        assert page.items == []