refactor: 拆分 3 个过大 ACP 文件为模块化子文件（每个 <500 行）

通过 4 阶段 workflow（分析 → 计划 → 重构 → 验证）将 3 个超大的 ACP
源文件拆分为 28 个模块化子文件，每个均严格小于 500 行，且完整保留
所有公共 API（barrel 模式重导出）。

变更概要：
- packages/acp-link/src/server.ts: 1800 → 20 行（barrel），新增 11 个子模块
  （server/types、payload-decode、permission-mode、runtime-state、dispatch、
  handlers-agent、handlers-session、acp-client、client-send、start-server、
  testing-internals）
- src/services/acp/agent.ts: 1297 → 33 行（barrel），新增 9 个子模块
  （agent/AcpAgent、sessionTypes、permissionMode、configOptions、promptQueue、
  internalAccessors、createSessionMethod、sessionLifecycle、promptFlow）
- src/services/acp/bridge.ts: 1516 → 29 行（barrel），新增 8 个子模块
  （bridge/types、paths、contentBlocks、toolInfo、toolResults、modelUsage、
  notifications、forwarding）

验证：
- bun run precheck 全通过（typecheck + lint + 5851 tests）
- ACP service tests: 176 pass / 0 fail
- ACP link tests: 47 pass / 0 fail
- 所有外部消费者（entry.ts、permissions.ts、__tests__/）的 import 路径不变
- 测试文件零修改

迁移计划详见 docs/acp-refactor-plan.md。

Co-Authored-By: glm-5.2 <zai-org@claude-code-best.win>

packages/acp-link/src/server/testing-internals.ts

@@ -0,0 +1,65 @@
+import type { ChildProcess } from 'node:child_process'
+import * as acp from '@agentclientprotocol/sdk'
+import type { WSContext } from 'hono/ws'
+import type { JsonRpc2ClientMessage } from '../ws-message.js'
+import { dispatchClientMessage, dispatchJsonRpcMessage } from './dispatch.js'
+import { clients, setDefaultPermissionMode } from './runtime-state.js'
+import { createClientState, type ProxyMessage } from './types.js'
+
+export function assertTestingInternalsEnabled(): void {
+  if (process.env.ACP_LINK_TEST_INTERNALS === '1') {
+    return
+  }
+
+  throw new Error(
+    'acp-link test internals are disabled outside test execution.',
+  )
+}
+
+export const __testing = {
+  dispatchClientMessage(ws: WSContext, data: unknown): Promise<void> {
+    assertTestingInternalsEnabled()
+    return dispatchClientMessage(ws, data as ProxyMessage)
+  },
+  dispatchJsonRpcMessage(ws: WSContext, data: unknown): Promise<void> {
+    assertTestingInternalsEnabled()
+    return dispatchJsonRpcMessage(ws, data as JsonRpc2ClientMessage)
+  },
+  registerClient(
+    ws: WSContext,
+    state: {
+      connection?: unknown
+      process?: ChildProcess | null
+      sessionId?: string | null
+      clientInfo?: { name: string; version: string }
+      clientCapabilities?: Record<string, unknown>
+      jsonRpc?: boolean
+    },
+  ): () => void {
+    assertTestingInternalsEnabled()
+    const full = createClientState()
+    full.process = state.process ?? null
+    full.connection = (state.connection ??
+      null) as acp.ClientSideConnection | null
+    full.sessionId = state.sessionId ?? null
+    if (state.clientInfo) full.clientInfo = state.clientInfo
+    if (state.clientCapabilities)
+      full.clientCapabilities = state.clientCapabilities
+    if (typeof state.jsonRpc === 'boolean') full.jsonRpc = state.jsonRpc
+    clients.set(ws, full)
+    return () => {
+      clients.delete(ws)
+    }
+  },
+  getClientSessionId(ws: WSContext): string | null | undefined {
+    assertTestingInternalsEnabled()
+    return clients.get(ws)?.sessionId
+  },
+  setDefaultPermissionMode(mode: string | undefined): () => void {
+    assertTestingInternalsEnabled()
+    const previous = setDefaultPermissionMode(mode)
+    return () => {
+      setDefaultPermissionMode(previous)
+    }
+  },
+}