fix: multi-bot accountId isolation in shared gateway

[Jerry-Xin]

Problem

When multiple bots share the same OpenClaw Gateway process and are members of the same group, messages/files sent by Bot A could be sent from Bot B's identity (accountId isolation failure).

Root Cause

_groupToAccount (Map<string, string>) used a last-write-wins strategy — whichever bot last processed a group message overwrote the mapping. The handleAction correction logic then replaced the caller's correct accountId with the wrong one.

Trigger Path

1. Bot B receives a group message → registerGroupToAccount(groupNo, 'bot-b') overwrites Bot A's entry
2. Bot A sends a message via message tool → handleAction calls resolveAccountForGroup(groupNo) → returns 'bot-b'
3. Bot A's correct accountId gets "corrected" to Bot B → message sent with Bot B's botToken

Changes

• Change _groupToAccount from Map<string, string> to Map<string, Set<string>> (1:N mapping)
• registerGroupToAccount now adds to a Set instead of overwriting
• resolveAccountForGroup returns undefined when multiple bots share a group (ambiguous)
• handleAction correction only applies when a single bot definitively owns the group
• 8 new test cases covering single-bot, multi-bot, and handleAction correction scenarios

Testing

• All 310 tests pass (including 8 new isolation tests)
• Verified on live deployment: both text and file attachments sent by correct bot identity in multi-bot group

Fixes: dmwork-org/dmwork-PM#183

[yujiawei]

CI Fix — Type Check Errors

核心逻辑没问题 👍，就是测试文件有两个类型问题导致 CI 挂了：

1. ctx 缺少 channel 属性

ChannelMessageActionContext 要求 channel 字段，三个 handleAction 测试的 ctx 都要补上：

const ctx = {
  accountId: "wrongBot",
  action: "send",
  channel: "dmwork",  // ← 补这个
  // ... 其余不变
};

2. dmworkPlugin.actions 可能为 undefined

三处 dmworkPlugin.actions!.handleAction(ctx) 需要加非空断言 !（或者统一在 describe 外面断言一次）：

// 方式 A：每次调用加 !
await dmworkPlugin.actions!.handleAction(ctx);

// 方式 B：describe 开头断言一次（推荐）
const handleAction = dmworkPlugin.actions?.handleAction;
assert(handleAction, "actions should be defined");

两处改完就能过 CI。

[yujiawei]

CI green ✅ 310/310 tests pass. 逻辑正确，类型问题已修。