[backplane][foundation] 抽取 _as_mapping 通用 Mapping 辅助 helper

[beachspainc]

现状与证据

_as_mapping 在 apps/** 与 src/cobnet/** 中被大量重复定义，当前语义高度一致（多数为 isinstance(value, Mapping) 检查后返回原值/None）。这是典型可抽离到 src/cobnet/backplane 的通用类型辅助函数。

命中清单（剔除 apps/google/**）：

1. apps/agentic/ports/shim/session_watch.py:27
2. apps/client/sdk/agent/roster/hub.py:34
3. apps/client/sdk/docs/docspec.py:111
4. apps/client/sdk/docs/tools.py:49
5. apps/client/sdk/shared/context/hub.py:26
6. apps/client/sdk/toolbox/hub.py:6693
7. apps/client/sdk/tools/catalog/hub.py:24
8. apps/client/sdk/workspace/backplane/views/context.py:41
9. apps/client/sdk/workspace/backplane/views/taskboard.py:21
10. apps/client/sdk/workspace/daemon/discovery.py:29
11. apps/client/sdk/workspace/mcp/server/app.py:127
12. apps/client/sdk/workspace/service/hub.py:31
13. apps/rag/query_params.py:53
14. apps/server/controller/backplane/context.py:31
15. apps/server/controller/backplane/toolbox.py:58
16. apps/server/controller/knowledge/hub.py:28
17. apps/server/controller/workspace/http.py:21
18. apps/server/controller/workspace/hub.py:28
19. apps/server/daemon/services/llm/usage.py:33
20. apps/server/gateway/llm/providers.py:89
21. apps/server/gateway/llm/tool_policy.py:68
22. apps/server/gateway/openai/events.py:16
23. apps/server/gateway/openai/tool_calls.py:21
24. apps/server/session/backplane.py:19
25. apps/server/state/agent/review.py:12
26. apps/server/state/agent/roster.py:22
27. apps/server/state/agent/welcome.py:23
28. apps/shared/profile/address.py:10
29. apps/shared/toolbox/command_hub.py:27
30. apps/shared/workspace/registry.py:33
31. apps/tool/shims/git_shim.py:381
32. apps/tool/workspace/daemon/ensure.py:120
33. src/cobnet/driver/backplane/pump.py:31
34. src/cobnet/protocol/task/action.py:40
35. src/cobnet/protocol/tool/request.py:16
36. src/cobnet/protocol/worker/artifact_store.py:18
37. src/cobnet/protocol/worker/job_spec.py:24
38. src/cobnet/protocol/worker/request.py:30

此外存在两类变体，建议保持显式参数化分流：

• return value if isinstance(value, Mapping) and not isinstance(value, (str, bytes)) else None（src/cobnet/executor/runner/backend/node/*）
• return value if isinstance(value, dict) or {} else {}（apps/vcs/service.py:721、src/cobnet/agent/*/reducer.py）

建议

• 在 src/cobnet/backplane 建立共享 helper（例如 as_mapping），默认语义为 Mapping -> value，否则 None。
• 仅保留 2 个参数化可选分支（coerce_empty、exclude_text_types）对特例（如 str/bytes）进行兼容。
• 统一替换上述 38 处重复实现并清理冗余。

验收标准

• 新增 src/cobnet/backplane 通用 helper 并补充单测。
• apps/** 和 src/cobnet/** 内上述重复定义全部切到统一 helper（Google 特例除外）。
• 保持既有行为兼容，并为两类特例写回归测试。