feat(mcp): correlated subscription streams (MCP 2026-07-28 subscriptions/listen)

[topcheer]

Summary

Implements the client side of the MCP 2026-07-28 subscription revision (subscriptions/listen): correlated notification streams that make list-change notifications attributable to a specific connection when an agent talks to several MCP servers concurrently.

Changes

• internal/mcp/subscriptions.go (new): full Subscription lifecycle —
  • ListenSubscriptions sends subscriptions/listen with the exact change set ggcode needs and waits for the spec notifications/subscriptions/acknowledged control message, which carries the granted subset.
  • Graceful stream termination and client-side cancel via notifications/cancelled referencing the listen request id.
  • Protocol MUST gating: notifications carrying a _meta subscription id that is unknown or unacknowledged are dropped, so a buggy server cannot poison cache invalidation; valid correlated traffic flows through the unchanged legacy dispatch.
  • EnableModernSubscriptions self-detects: a legacy server -32601 latches a permanent per-connection downgrade.
• internal/mcp/client.go: subscription registry, a single routing hook at the top of the notification dispatch path (zero behavior change for uncorrelated traffic), Abort closes all subscriptions.
• internal/plugin/mcp_loader.go: auto-enable on the shared connect path so auto-reconnect re-opens the stream.
• Tests: pipe-based NDJSON stdio fake-server harness (ack/graceful, downgrade latching with wire-count, cancelled propagation, MUST-gating matrix, abort teardown, filter-diff). Race-clean under -race -count=2. HTTP mocks now echo real request ids and answer the probe with spec -32601.
• Docs: new Subscription Streams section in docs/guide/mcp.md.

Compatibility

• stdio: streaming listen supported; HTTP probes and gracefully downgrades; ws explicitly rejected; legacy servers unchanged after one probe.

🤖 Generated with ggcode

[topcheer]

复审通过 ✅（三重点核销）

① 与 #2480 关联升级——叠加非替代：listen 流通知 correlate 后剥离 _meta 转发到既有 legacy handler（工具/资源刷新链零改动收 correlation 收益）；#2480 的 per-URI 订阅在 modern 服务器上由 filter.ResourceSubscriptions 承载（升级面），legacy 服务器维持原路径——单订阅单流（modernSub 唯一+活流 short-circuit），无多 listen 扇出需求面（filter 合并即全量）。

② correlation ID 作用域：subscriptionId=listen 请求的 JSON-RPC id（服务器 echo 回）——client 实例内 nextID 单调分配=天然唯一；MUST-gating：未 ack/未知 subscriptionId 通知=协议违反 drop+log（服务器不得先发后 ack 的客户端侧强制）——乱序攻击面闭合。

③ 与 #2510 stateless 兼容：-32601 永久降级（对齐 GET 流 405 降级形态）——legacy 服务器零重复调用；listen 长流是请求内长响应（stdio waiter/HTTP SSE POST 内嵌）非跨请求会话——stateless 的 per-request _meta 与长流响应正交；15s ack 钳+取消走既有 notifyCancelled（复用 #2490 管道）。WS 显式不支持（legacy bridge 域声明）。

CI 全绿。可合并。