[KYUUBI #7379][2b/4] Data Agent Engine: agent runtime, middleware, and OpenAI provider

[wangzhigang1999]

Summary

Part 2b of 4 for the Data Agent Engine (umbrella). Stacked on #7380 and PR 2a, please review those first.

• ReactAgent — ReAct loop with streaming LLM interaction and configurable max iterations
• ConversationMemory — per-session multi-turn context management
• Middleware pipeline: ApprovalMiddleware (STRICT/NORMAL/AUTO_APPROVE), LoggingMiddleware
• OpenAiProvider — wires ReactAgent + tools + middleware for any OpenAI-compatible LLM service
• MockLlmProvider — enables full pipeline integration testing without a real LLM

Note: Stacked PR. After prior PRs merge, this will be rebased to show only incremental diff.

Test plan

• ReactAgentBuilderTest — builder validation
• ConversationMemoryTest — message management, defensive copy
• ApprovalMiddlewareTest — approve/deny, timeout, risk-level routing, cancelAll
• LoggingMiddlewareTest — event pass-through, truncation
• DataAgentMockLlmSuite — full tool-call pipeline via JDBC with MockLlmProvider
• DataAgentE2ESuite — end-to-end with real LLM (CI-optional)
• ApprovalWorkflowIntegrationSuite — approval workflow via JDBC