[data, vlm] fix: skip overlong Kimi VLM samples

[kaimo455]

Summary

• avoid running Kimi media processing when a multimodal sample has no images or videos
• only consume <image> / <video> placeholders when corresponding media is present, so text-only multimodal samples can contain those literals
• discard overlong Kimi VLM samples via warning + skipped encode result when enable_discard_sample is set, respecting max_packed_tokens for packed data

Adapted from internal iCode review 120864956 patchset 2.

Verification

• final GitHub blobs match iCode patchset 2 for both modified files
• git diff --check origin/master..HEAD
• python -m compileall -q loongforge/data/kimi_k25_plugin.py loongforge/data/multimodal/kimi_task_encoder.py