Fix the bug in plan notebook serialization

[DavdGao]

AgentScope Version

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Description

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Checklist

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• Code has been formatted with pre-commit run --all-files command
• All tests are passing
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[Copilot]

Pull Request Overview

Fixes a bug in plan notebook serialization where the custom_to_json function was returning a JSON string instead of a dictionary, causing serialization issues.

• Changed model_dump_json() to model_dump() in the PlanNotebook class for proper state serialization
• Added comprehensive test case to verify serialization and deserialization functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/agentscope/plan/_plan_notebook.py	Fixed serialization bug by changing model_dump_json() to model_dump()
tests/plan_test.py	Added test case to verify plan notebook serialization works correctly

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