fix(agent): honor returnDirect routing

[6mmLIU]

Summary

Fixes the ReactAgent returnDirect routing behavior reported in #4427.

jump_to is a transient hook routing command, but it was stored in graph state. After a returnDirect tool set jump_to=end, the checkpoint for that turn could carry the stale command into a later invocation on the same thread and short-circuit before the model ran. ReactAgent also did not install the existing return-direct model hook by default, so @Tool(returnDirect = true) was not honored unless users wired the hook manually.

This change:

• registers ReturnDirectModelHook automatically for tool-enabled ReactAgent instances unless an equivalent hook is already present
• consumes and removes jump_to when routing so one-shot hook decisions do not leak through checkpoints or later turns
• routes tool-node output back through the before-model hook chain, letting ReturnDirectModelHook end direct tool responses while normal tools still return to the model
• adds a mock ChatModel regression test covering direct tools, same-thread follow-up turns, and normal tool routing

Fixes #4427

Tests

• ./mvnw -pl :spring-ai-alibaba-agent-framework -am -Dtest=ReactAgentReturnDirectRoutingTest test

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