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/app/services/c2/fake.py

@@ -1,6 +1,7 @@
 """Deterministic in-memory C2 adapter — used when MIMIC_C2_ADAPTER=fake.
 
 Intended for integration tests and local development without a live Mythic instance.
+Task state is per-instance so parallel tests don't interfere with each other.
 """
 from __future__ import annotations
 
@@ -12,6 +13,7 @@ from backend.app.services.c2.adapter import (
     C2TaskStatus,
 )
 
+# Three fixed callbacks the test suite can pin against.
 _FAKE_CALLBACKS = [
     C2Callback(
         display_id=1,
@@ -21,14 +23,34 @@ _FAKE_CALLBACKS = [
         domain="LAB",
         last_checkin="2026-06-10T00:00:00Z",
     ),
+    C2Callback(
+        display_id=2,
+        active=True,
+        host="SERVER-DC01",
+        user="svc_backup",
+        domain="LAB",
+        last_checkin="2026-06-10T00:01:00Z",
+    ),
+    C2Callback(
+        display_id=3,
+        active=True,
+        host="LAPTOP-RT",
+        user="admin",
+        domain="LAB",
+        last_checkin="2026-06-10T00:02:00Z",
+    ),
 ]
 
-_FAKE_TASKS: dict[int, dict] = {}
-_next_task_id = 100
-
 
 class FakeAdapter(C2Adapter):
-    """In-memory adapter with deterministic behaviour."""
+    """In-memory adapter with deterministic behaviour.
+
+    Each instance starts with an empty task store and display_ids from 1000.
+    """
+
+    def __init__(self) -> None:
+        self._tasks: dict[int, dict] = {}
+        self._next_task_id = 1000
 
     def test_connection(self) -> C2Health:
         return C2Health(ok=True)
@@ -42,10 +64,9 @@ class FakeAdapter(C2Adapter):
         command: str,
         params: str | None = None,
     ) -> int:
-        global _next_task_id
-        tid = _next_task_id
-        _next_task_id += 1
-        _FAKE_TASKS[tid] = {
+        tid = self._next_task_id
+        self._next_task_id += 1
+        self._tasks[tid] = {
             "display_id": tid,
             "callback_display_id": callback_display_id,
             "command": command,
@@ -57,7 +78,7 @@ class FakeAdapter(C2Adapter):
         return tid
 
     def get_task(self, task_display_id: int) -> C2TaskStatus:
-        task = _FAKE_TASKS.get(task_display_id)
+        task = self._tasks.get(task_display_id)
         if task is None:
             return C2TaskStatus(display_id=task_display_id, status="unknown", completed=False)
         return C2TaskStatus(
@@ -67,7 +88,7 @@ class FakeAdapter(C2Adapter):
         )
 
     def get_task_output(self, task_display_id: int) -> str:
-        task = _FAKE_TASKS.get(task_display_id)
+        task = self._tasks.get(task_display_id)
         if task is None:
             return ""
         return task.get("output") or ""
@@ -79,7 +100,7 @@ class FakeAdapter(C2Adapter):
         page_size: int = 25,
     ) -> C2TaskPage:
         items = [
-            t for t in _FAKE_TASKS.values()
+            t for t in self._tasks.values()
             if t["callback_display_id"] == callback_display_id
         ]
         start = (page - 1) * page_size