[WIP] fix(trainer,checkpoint,explorer): resolve Qwen3.5/vLLM compatibility bugs and harden runtime paths

[chenyushuo]

Description

1. Fix Qwen3.5 sequence-parallel training bugs.
2. Fix Qwen3.5 multimodal training bugs.
3. Fix incorrect Qwen3.5 checkpoint parameter naming when saving with Transformers 5.4.0-5.5.4.
4. Add freeze_vision_tower support.
5. Fix compatibility issues with vLLM 0.20.
6. Fix a bug in Experience serialization.
7. Fix the condition for skipping TrainerConfigValidator checks.
8. Improve explorer robustness by safely handling missing rollout coordinator instead of hard-asserting.
9. Propagate checkpoint_job_dir into workflow/taskset runtime arguments.
10. Improve FSDP worker initialization and logging behavior for better stability and observability.
11. Apply typo and minor message fixes.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has passed all tests
• Docstrings have been added/updated in Google Style
• Documentation has been updated
• Code is ready for review