fix(finetune): respect later-turn image placeholders

[primorLee]

Summary

• scan all conversation turns when replacing the single-image placeholder
• preserve the existing first-turn fallback when no placeholder is present
• add regression coverage for later-turn placement and the fallback path

Context

This carries the focused finetuning fix from OpenBMB/MiniCPM-V#1134 into the actively maintained Cookbook, as requested by the MiniCPM-V maintainer in that PR.

Test

• python -X utf8 -m unittest finetune/official/test_dataset.py (2 passed)
• ruff check finetune/official/test_dataset.py
• ruff format --check finetune/official/test_dataset.py

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