feat(runner): 注入 tool_dispatch_factory/prompt_builder_factory + fail-fast

Runner.__init__ 新增 2 个可选参数:
- tool_dispatch_factory: 工具调度工厂
- prompt_builder_factory: prompt 构建工厂

infer/eval/train 模式缺少工厂时 fail-fast 抛 ValueError。
_make_tool_dispatch_fn/_make_prompt_builder 优先使用注入工厂。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_harness_runner.py

@@ -14,6 +14,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 from app.harness.runner import (
+    Runner,
     _apply_batch_correctness,
     _batch_from_ids,
     _build_comparison_pairs,
@@ -680,3 +681,94 @@ class TestCooldownDecrement:
 
         cooldown = {t: n - 1 for t, n in cooldown.items() if n - 1 > 0}
         assert cooldown == {}
+
+
+# =========================================================================
+# factory 注入（Task 4）
+# =========================================================================
+
+
+class TestRunnerFactoryInjection:
+    """Runner 构造时 tool_dispatch_factory / prompt_builder_factory 注入检查。"""
+
+    @staticmethod
+    def _base_config(tmp_path: Path, *, mode: str = "infer", **overrides):
+        """构造 RunConfig，所有必填字段都给默认值。"""
+        from app.harness.config import RunConfig
+
+        defaults = {
+            "workspace_dir": tmp_path,
+            "store_dir": tmp_path,
+            "mode": mode,
+            "concurrency": 1,
+            "max_steps": 5,
+            "skill_mode": "none",
+            "n_samples": 0,
+            "questions": "benchmarks/Video-MME",
+            "skills_version": "v1",
+            "prompts_version": "v1",
+            "epochs": 1,
+            "diag_size": 10,
+            "diag_correct_ratio": 0.5,
+            "val_size": 24,
+            "val_correct_ratio": 0.5,
+            "edit_budget_start": 5,
+            "edit_budget_end": 2,
+            "batch_size": 5,
+            "min_class_per_batch": 2,
+            "eval_min_per_class": 2,
+            "early_stop_patience": 3,
+            "test_size": 10,
+            "use_slow_momentum": False,
+            "gate_e_confirm": 20.0,
+            "gate_e_provisional": 3.0,
+            "gate_w_net_min": 2,
+            "gate_delta_min": 0.02,
+            "gate_lambda_dir": -0.642,
+            "gate_e_rollback": 10.0,
+            "gate_block": 8,
+            "gate_n_max": 40,
+            "gate_p_low": 0.05,
+            "gate_p_high": 0.95,
+            "gate_probe_quota": 0.2,
+            "gate_gamma_decay": 0.9,
+            "gate_cooldown_steps": 2,
+            "gate_guard_err": 0.10,
+            "skill_update_mode": "patch",
+            "appendix_consolidate_threshold": 6,
+        }
+        defaults.update(overrides)
+        return RunConfig(**defaults)
+
+    def test_infer_mode_missing_factory_raises(self, tmp_path: Path) -> None:
+        """infer 模式缺少工厂时抛出 ValueError。"""
+        config = self._base_config(tmp_path, mode="infer")
+        with pytest.raises(ValueError, match="tool_dispatch_factory"):
+            Runner(
+                config,
+                llm=MagicMock(),
+                evolve_llm=MagicMock(),
+                vlm=MagicMock(),
+                telemetry=MagicMock(),
+            )
+
+    def test_diagnose_mode_allows_none_factory(self, tmp_path: Path) -> None:
+        """diagnose 模式不需要工厂，允许 None。"""
+        ws = tmp_path / "ws"
+        ws.mkdir()
+        (ws / "manifest.json").write_text(
+            '{"name":"ws","created_at":"","store":"../store",'
+            '"current":{"videos":"v","questions":"q","skills":"s","prompts":"p"},'
+            '"history":[]}'
+        )
+        config = self._base_config(tmp_path, mode="diagnose", workspace_dir=ws, run_id="test_run")
+        # 不应抛出 ValueError
+        runner = Runner(
+            config,
+            llm=MagicMock(),
+            evolve_llm=MagicMock(),
+            vlm=MagicMock(),
+            telemetry=MagicMock(),
+        )
+        assert runner._tool_dispatch_factory is None
+        assert runner._prompt_builder_factory is None