feat(mcp): Top5#3 对齐 MCP Go SDK——协议版本协商 + list_changed 差量刷新 + keepalive + Streamable HTTP

- 协议版本协商：initialize 先发最高版本(2025-11-25)并声明能力集合，遇
  UnsupportedProtocolVersionError(-32022)/服务器不支持版本 → negotiateMutuallySupportedVersion
  求出双方最高共同版本降级重试(最多2次)；无共同版本→ErrUnsupportedProtocolVersion。
  协商结果记 negotiatedVersion，HTTP 传输回填 Mcp-Protocol-Version 头。
- tools/list_changed 订阅：capabilities 声明 tools.listChanged:true；传输读无 id 帧路由到
  handleServerNotification；refreshPending 原子旗标合并突发 + 新 goroutine 调 onToolListChanged
  (避免 stdio lineRPC 重入自锁)；Registry 真差量刷新(重新 DiscoverTools→移除消失/注册新增，
  跳过同名冲突保外来工具)。
- keepalive：WithKeepAlive(interval,failures) 起 goroutine 定时 Call(ping)，连续失败≥阈值关
  session(默认3,可配)；服务器不支持 ping(-32601)停 keepalive 不误关；ctx+Close 取消不泄漏。
- Streamable HTTP 传输：newTransport 支持 TransportType=http|streamable(保留 stdio/SSE 推断，
  空值走旧逻辑不破坏)；Call POST+Accept json/event-stream 响应可单 JSON 或 SSE 流找匹配 id；
  Mcp-Session-Id 会话 + 独立 GET SSE 流收 server 通知；config 读 MCP_TRANSPORT。
- schema 缓存判定不做：客户端透传 map[string]any 无重复编译，SDK SchemaCache 是服务端
  按 Go 类型反射场景，客户端无对应。

go build/vet/test 全绿(count=2 无 flaky) + windows/darwin 交叉编译。

internal/capability/mcp/helpers_test.go

@@ -2,23 +2,40 @@ package mcp
 
 import (
 	"context"
+	"fmt"
 	"sync"
 )
 
 // fakeTransport is a scripted Transport for tests. It records starts, closes and
 // the methods it was asked to call so assertions can verify the client's usage.
+// It also lets tests script the initialize handshake, simulate server→client
+// notifications and control the ping result.
 type fakeTransport struct {
 	startErr error
 	closeErr error
 	initErr  error
 	listErr  error
 	callErr  error
+	pingErr  error
 	tools    []ToolInfo
 
-	mu      sync.Mutex
-	starts  int
-	closes  int
-	methods []string
+	// initErrors is a per-call queue of initialize errors. When non-empty one
+	// entry is consumed per initialize call; once exhausted the handshake
+	// succeeds. initErr is returned for every call when initErrors is empty.
+	initErrors []error
+	// initVersions records the protocolVersion sent in each initialize call.
+	initVersions []string
+	// initResponseVersion is the protocolVersion echoed on a successful
+	// initialize; default 2024-11-05.
+	initResponseVersion string
+
+	mu          sync.Mutex
+	starts      int
+	closes      int
+	methods     []string
+	pings       int
+	onNotifyFn  func(method string)
+	activeTools []ToolInfo
 }
 
 // Start records a start.
@@ -26,29 +43,42 @@ func (f *fakeTransport) Start(_ context.Context) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	f.starts++
+	f.activeTools = f.tools
 	return f.startErr
 }
 
-// Call answers the three MCP methods the client issues.
-func (f *fakeTransport) Call(_ context.Context, method string, _ any, result any) error {
+// Call answers the MCP methods the client issues.
+func (f *fakeTransport) Call(_ context.Context, method string, params any, result any) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	f.methods = append(f.methods, method)
 	switch method {
 	case "initialize":
+		if p, ok := params.(initializeParams); ok {
+			f.initVersions = append(f.initVersions, p.ProtocolVersion)
+		}
+		if len(f.initErrors) > 0 {
+			err := f.initErrors[0]
+			f.initErrors = f.initErrors[1:]
+			return err
+		}
 		if f.initErr != nil {
 			return f.initErr
 		}
 		if r, ok := result.(*initializeResult); ok {
 			r.ServerInfo = serverInfo{Name: "fake-server", Version: "1.0"}
-			r.ProtocolVersion = protocolVersion
+			r.ProtocolVersion = f.initResponseVersion
+			if r.ProtocolVersion == "" {
+				r.ProtocolVersion = protocolVersion20241105
+			}
+			f.activeTools = f.tools
 		}
 	case "tools/list":
 		if f.listErr != nil {
 			return f.listErr
 		}
 		if r, ok := result.(*listToolsResult); ok {
-			for _, t := range f.tools {
+			for _, t := range f.activeTools {
 				r.Tools = append(r.Tools, toolSpec{Name: t.Name, Description: t.Description, InputSchema: t.InputSchema})
 			}
 		}
@@ -59,6 +89,11 @@ func (f *fakeTransport) Call(_ context.Context, method string, _ any, result any
 		if r, ok := result.(*callToolResult); ok {
 			r.Content = []contentBlock{{Type: "text", Text: "hello"}}
 		}
+	case "ping":
+		f.pings++
+		if f.pingErr != nil {
+			return f.pingErr
+		}
 	}
 	return nil
 }
@@ -71,6 +106,25 @@ func (f *fakeTransport) notify(_ context.Context, method string, _ any) error {
 	return nil
 }
 
+// setOnNotification records the client's server→client notification handler so
+// tests can deliver one via notifyServer.
+func (f *fakeTransport) setOnNotification(fn func(method string)) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.onNotifyFn = fn
+}
+
+// notifyServer simulates a server→client notification (e.g.
+// notifications/tools/list_changed) delivered to the client's handler.
+func (f *fakeTransport) notifyServer(method string) {
+	f.mu.Lock()
+	fn := f.onNotifyFn
+	f.mu.Unlock()
+	if fn != nil {
+		fn(method)
+	}
+}
+
 // Close records a close.
 func (f *fakeTransport) Close() error {
 	f.mu.Lock()
@@ -97,3 +151,38 @@ func (f *fakeTransport) hasMethod(method string) bool {
 	}
 	return false
 }
+
+// initSentVersions returns the protocol versions the client requested across all
+// initialize calls.
+func (f *fakeTransport) initSentVersions() []string {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	out := make([]string, len(f.initVersions))
+	copy(out, f.initVersions)
+	return out
+}
+
+// pingCount returns the recorded ping request count.
+func (f *fakeTransport) pingCount() int {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.pings
+}
+
+// setTools replaces the tool list served on subsequent tools/list calls.
+func (f *fakeTransport) setTools(tools []ToolInfo) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.tools = tools
+	f.activeTools = tools
+}
+
+// unsupportedProtocolError builds a JSON-RPC error that rejects the requested
+// protocol version and advertises the supported set, as a real MCP server does.
+func unsupportedProtocolError(supported ...string) error {
+	return fmt.Errorf("mcp initialize: %w: %w", ErrRPC, &rpcError{
+		Code:    codeUnsupportedProtocolVersion,
+		Message: "unsupported protocol version",
+		Data:    map[string]any{"supported": supported},
+	})
+}