* fix: resolve issue #650 - repair missing step_type fields in Plan validation
- Add step_type repair logic to validate_and_fix_plan() to auto-infer missing step_type
- Infer as 'research' when need_search=true, 'processing' when need_search=false
- Add explicit CRITICAL REQUIREMENT section to planner.md emphasizing step_type mandatory for every step
- Include validation checklist and examples showing both research and processing steps
- Add 23 comprehensive unit tests for validate_and_fix_plan() covering all scenarios
- Add 4 integration tests specifically for Issue #650 with actual Plan validation
- Prevents Pydantic ValidationError: 'Field required' for missing step_type
* Update tests/unit/graph/test_plan_validation.py
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* Update tests/unit/graph/test_plan_validation.py
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* update the planner.zh_CN.md with recent changes of planner.md
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
