[KYUUBI #7529][DATA-AGENT] Support cross-server session routing

[wangzhigang1999]

Why are the changes needed?

In a multi-server deployment, Data Agent requests may reach a server that does not own the local session and fail with session not found.

This patch stores the session-to-engine route in the HA backend, allowing any server to attach to the existing engine session without resetting its state.

Related to #7529.

How was this patch tested?

• HA and REST test suites: 39 tests passed.
• All 17 affected Maven modules built successfully.
• Tested two Kyuubi Servers with ZooKeeper, covering cross-server chat, approval, close, concurrent access, state retention, authorization, and stale route cleanup.

Was this patch authored or co-authored using generative AI tooling?

Assisted-by: OpenAI Codex with GPT-5

[wangzhigang1999]

Manually tested on a two-server Kubernetes deployment:

• Opened a Data Agent session on one server, then accessed, chatted, approved tool calls, and closed it through the other server.
• Verified conversation state, user isolation, stale-route cleanup, and real model/SQL execution.
• Ran four concurrent sessions and 500 cross-server requests without failures or resource leaks.

Server and local Engine failure recovery remains outside this PR.