feat: 向量记忆 RAG、工具市场、SSE 流式响应、前端集成与测试覆盖

- 新增 embedding_service（语义检索）、knowledge_service（RAG）、text_chunker、document_parser
- 新增 tool_registry（自定义工具注册表）并完善工具市场 API（CRUD + code/http 执行）
- 新增 agent_vector_memory / knowledge_base 模型及对应数据库表
- 实现 SSE 流式响应与 Agent 预算控制
- AgentChat.vue 集成 MainLayout 导航布局
- 完善测试体系：7 个新测试文件共 110 个测试覆盖
- 修复 conftest.py SQLite 内存数据库连接隔离问题

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -13,5 +13,7 @@ from app.models.permission import Role, Permission, WorkflowPermission, AgentPer
 from app.models.alert_rule import AlertRule, AlertLog
 from app.models.persistent_user_memory import PersistentUserMemory
 from app.models.agent_llm_log import AgentLLMLog
+from app.models.agent_vector_memory import AgentVectorMemory
+from app.models.knowledge_base import KnowledgeBase, Document, DocumentChunk
 
-__all__ = ["User", "Workflow", "WorkflowVersion", "Agent", "Execution", "ExecutionLog", "ModelConfig", "DataSource", "WorkflowTemplate", "TemplateRating", "TemplateFavorite", "NodeTemplate", "Role", "Permission", "WorkflowPermission", "AgentPermission", "AlertRule", "AlertLog", "PersistentUserMemory", "AgentLLMLog"]
+__all__ = ["User", "Workflow", "WorkflowVersion", "Agent", "Execution", "ExecutionLog", "ModelConfig", "DataSource", "WorkflowTemplate", "TemplateRating", "TemplateFavorite", "NodeTemplate", "Role", "Permission", "WorkflowPermission", "AgentPermission", "AlertRule", "AlertLog", "PersistentUserMemory", "AgentLLMLog", "AgentVectorMemory", "KnowledgeBase", "Document", "DocumentChunk"]