Add fallback shim for agents dependency

[zhongkaifu]

Summary

• add an internal shim so planner imports succeed when the optional agents package is missing or incompatible
• surface a clear warning and runtime error guidance instead of failing on import when AsyncOpenAI is unavailable

Testing

• pytest tests/test_condition_branches.py tests/test_repair_fallback_preserve.py tests/test_repair_patch_output.py tests/test_rule_based_repair_pipeline.py tests/test_rule_based_repairs.py tests/test_workflow_builder_update.py

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