fix: clear jump_to state after each MessagesModelHook execution to prevent residual values

[zhexueqi]

Problem

When a MessagesModelHook overrides canJumpTo() to return List.of(JumpTo.end), the framework registers a conditional edge that reads the jump_to field from state to decide routing.

Previously, if beforeModel() or afterModel() returned new AgentCommand(messages) (no jump intent), the jump_to field was not written to state. This left any residual value from a previous execution intact, causing the workflow to terminate prematurely even when the hook did not intend to jump.

Root Cause

In MessagesModelHook.BeforeModelAction and AfterModelAction:

// Before fix - only writes when jumpTo != null
if (command.getJumpTo() != null) {
    result.put("jump_to", command.getJumpTo().name());
}

Fix

Always write jump_to to state. When there is no jump intent, explicitly set null to clear any stale value from prior executions.

// After fix - always clears or sets jump_to
result.put("jump_to", command.getJumpTo() != null ? command.getJumpTo().name() : null);
Fixes #4421

[CLAassistant]

All committers have signed the CLA.

[wangliuliu1115]

I've verified this PR fixes the issue described in #4421. Thanks for the quick fix!

Tested on my local environment with the fix/messages-model-hook-jump-to-residual branch and it works as expected.

Thanks again!

[robocanic]

@robocanic