feat(capability/mcp): MCP 客户端（stdlib JSON-RPC over stdio/SSE）+ Registry 自动注册/Unregister（registry 增 Remove）

internal/capability/mcp/rpc.go

@@ -0,0 +1,50 @@
+package mcp
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// rpcRequest is a JSON-RPC 2.0 request. Notifications omit ID so the field is
+// tagged omitempty. Params is any so callers pass a struct or a JSON object.
+type rpcRequest struct {
+	JSONRPC string `json:"jsonrpc"`
+	ID      any    `json:"id,omitempty"`
+	Method  string `json:"method"`
+	Params  any    `json:"params,omitempty"`
+}
+
+// rpcResponse is a JSON-RPC 2.0 response. Result is kept raw until the caller
+// decodes it into the concrete result type.
+type rpcResponse struct {
+	JSONRPC string          `json:"jsonrpc"`
+	ID      any             `json:"id,omitempty"`
+	Result  json.RawMessage `json:"result,omitempty"`
+	Error   *rpcError       `json:"error,omitempty"`
+}
+
+// rpcError is a JSON-RPC error object. It satisfies error so it can be wrapped
+// and surfaced with %w.
+type rpcError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+	Data    any    `json:"data,omitempty"`
+}
+
+// Error implements error.
+func (e *rpcError) Error() string {
+	return fmt.Sprintf("code %d: %s", e.Code, e.Message)
+}
+
+// encodeRequest marshals a JSON-RPC request. An id of nil produces a
+// notification (no id field).
+func encodeRequest(id any, method string, params any) ([]byte, error) {
+	return json.Marshal(rpcRequest{JSONRPC: "2.0", ID: id, Method: method, Params: params})
+}
+
+// decodeIDString reports whether res carries the string id expected for a JSON
+// string id request.
+func decodeIDString(res rpcResponse, id string) bool {
+	s, ok := res.ID.(string)
+	return ok && s == id
+}