feat(backend): c2 poll-on-read + output mapping (sprint 8 M3)

- adapter.py: add completed_at field to C2TaskStatus dataclass
- mythic.py: implement get_task() (GraphQL task query) and
  get_task_output() (response query + decode_response_text concat)
- fake.py: deterministic state progression via per-instance call counter;
  get_task_output raises C2Error until completed
- mapping.py: apply_task_to_simulation() idempotent output mapper
  (mapping_applied anchor prevents double-writes)
- migration 0007: add mapping_applied BOOLEAN NOT NULL DEFAULT false to c2_task
- c2_task model: mapping_applied column added
- api/c2.py: GET /api/simulations/<id>/c2/tasks poll-on-read endpoint;
  refreshes incomplete tasks from C2, fetches output on completion,
  applies mapping, skips re-polling for completed tasks; best-effort
  (C2Error on individual task skipped, returns 200 with stale status)
- 51 new tests (396 total); pytest/ruff/mypy all green

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

backend/tests/test_c2_mapping.py

@@ -0,0 +1,97 @@
+"""Unit tests for apply_task_to_simulation() mapping helper."""
+from __future__ import annotations
+
+from datetime import UTC, datetime
+from unittest.mock import MagicMock
+
+from backend.app.services.c2.mapping import apply_task_to_simulation
+
+
+def _make_task(output: str | None = "whoami output", mapping_applied: bool = False) -> MagicMock:
+    task = MagicMock()
+    task.output = output
+    task.mapping_applied = mapping_applied
+    return task
+
+
+def _make_sim(execution_result: str | None = None) -> MagicMock:
+    sim = MagicMock()
+    sim.execution_result = execution_result
+    sim.updated_at = None
+    return sim
+
+
+class TestApplyTaskToSimulation:
+    def test_appends_output_to_empty_simulation(self):
+        task = _make_task(output="whoami output")
+        sim = _make_sim(execution_result=None)
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.execution_result == "whoami output"
+        assert task.mapping_applied is True
+
+    def test_appends_with_newline_separator(self):
+        task = _make_task(output="second result")
+        sim = _make_sim(execution_result="first result")
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.execution_result == "first result\nsecond result"
+
+    def test_idempotent_when_already_applied(self):
+        task = _make_task(output="some output", mapping_applied=True)
+        sim = _make_sim(execution_result="existing")
+
+        apply_task_to_simulation(task, sim)
+
+        # execution_result must not be modified.
+        assert sim.execution_result == "existing"
+
+    def test_no_op_when_output_is_empty_string(self):
+        task = _make_task(output="")
+        sim = _make_sim(execution_result="existing")
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.execution_result == "existing"
+        # Still marks mapping_applied so we don't revisit it.
+        assert task.mapping_applied is True
+
+    def test_no_op_when_output_is_none(self):
+        task = _make_task(output=None)
+        sim = _make_sim(execution_result="existing")
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.execution_result == "existing"
+        assert task.mapping_applied is True
+
+    def test_strips_trailing_newlines_from_existing(self):
+        """Existing execution_result with trailing newlines should not cause double blank lines."""
+        task = _make_task(output="new output")
+        sim = _make_sim(execution_result="old output\n\n")
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.execution_result == "old output\nnew output"
+
+    def test_updated_at_is_set_on_sim(self):
+        task = _make_task(output="something")
+        sim = _make_sim(execution_result=None)
+        before = datetime.now(UTC)
+
+        apply_task_to_simulation(task, sim)
+
+        assert sim.updated_at is not None
+        assert sim.updated_at >= before
+
+    def test_multiple_tasks_accumulate(self):
+        sim = _make_sim(execution_result=None)
+        tasks = [_make_task(output=f"result {i}") for i in range(3)]
+
+        for t in tasks:
+            apply_task_to_simulation(t, sim)
+
+        lines = sim.execution_result.split("\n")
+        assert lines == ["result 0", "result 1", "result 2"]