test: 补充单元测试覆盖率 29.9% → 36.6%

新建 18 个测试文件，追加 30 个已有测试文件，覆盖协议解析、
错误分类、CEL 表达式求值、YAML 反序列化、字节编码等纯函数。

plugins/services/neo4j_test.go

@@ -4,6 +4,7 @@ package services
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -32,3 +33,24 @@ func TestNeo4jUnauthorizedRequiresNeo4jBody(t *testing.T) {
 		t.Fatalf("testUnauthorizedAccess reported generic 200 as Neo4j: %#v", result)
 	}
 }
+
+func TestClassifyNeo4jErrorType(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want ErrorType
+	}{
+		{"nil", nil, ErrorTypeUnknown},
+		{"authentication failed", errors.New("authentication failed"), ErrorTypeAuth},
+		{"401 unauthorized", errors.New("401 unauthorized"), ErrorTypeAuth},
+		{"connection refused", errors.New("connection refused"), ErrorTypeNetwork},
+		{"unknown", errors.New("random neo4j error"), ErrorTypeUnknown},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := classifyNeo4jErrorType(tt.err); got != tt.want {
+				t.Errorf("classifyNeo4jErrorType() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}