feat: 添加 Agent 步骤事件与图片附件功能

- 新增 stepStarted/stepFinished 事件类型支持
- 前端实现图片附件上传和预览功能
- 后端增强工具结果存储和事件处理
- 完善相关单元测试和集成测试

backend/tests/unit/v1/agent/test_attachment_storage.py

@@ -0,0 +1,85 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+import v1.agent.attachment_storage as attachment_storage_module
+
+
+class _FakeBucket:
+    def __init__(self) -> None:
+        self.upload_calls: list[tuple[str, bytes, dict[str, str]]] = []
+        self.download_calls: list[str] = []
+
+    def upload(self, path: str, content: bytes, options: dict[str, str]) -> object:
+        self.upload_calls.append((path, content, options))
+        return {"path": path}
+
+    def download(self, path: str) -> object:
+        self.download_calls.append(path)
+        return b"ok"
+
+
+class _FakeStorage:
+    def __init__(self, bucket: _FakeBucket) -> None:
+        self._bucket = bucket
+
+    def from_(self, bucket: str) -> object:
+        del bucket
+        return self._bucket
+
+
+@pytest.mark.asyncio
+async def test_attachment_storage_rejects_unexpected_bucket(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    storage = attachment_storage_module.AgentAttachmentStorage()
+    monkeypatch.setattr(
+        attachment_storage_module.config.storage,
+        "bucket",
+        "allowed-bucket",
+    )
+
+    with pytest.raises(RuntimeError, match="Invalid attachment bucket"):
+        await storage.upload_bytes(
+            bucket="other-bucket",
+            path="agent-inputs/u/t/r/file.png",
+            content=b"data",
+            content_type="image/png",
+        )
+
+
+@pytest.mark.asyncio
+async def test_attachment_storage_accepts_configured_bucket(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    storage = attachment_storage_module.AgentAttachmentStorage()
+    fake_bucket = _FakeBucket()
+    fake_client = SimpleNamespace(storage=_FakeStorage(fake_bucket))
+    monkeypatch.setattr(
+        attachment_storage_module.config.storage,
+        "bucket",
+        "allowed-bucket",
+    )
+    monkeypatch.setattr(
+        attachment_storage_module.supabase_service,
+        "get_admin_client",
+        lambda: fake_client,
+    )
+
+    path = await storage.upload_bytes(
+        bucket="allowed-bucket",
+        path="agent-inputs/u/t/r/file.png",
+        content=b"data",
+        content_type="image/png",
+    )
+    payload = await storage.download_bytes(
+        bucket="allowed-bucket",
+        path=path,
+    )
+
+    assert path == "agent-inputs/u/t/r/file.png"
+    assert payload == b"ok"
+    assert len(fake_bucket.upload_calls) == 1
+    assert fake_bucket.download_calls == ["agent-inputs/u/t/r/file.png"]