[Bugfix][Rust Frontend][Multimodal] Align DeepSeek V4.1 and Kimi K3 media with rendered placeholders

[reidliu41]

Purpose

Rust chat rendering and multimodal extraction previously determined media
order independently.

DeepSeek V4.1 and Kimi K3 can reorder tool results to match the preceding
assistant tool-call order. The rendered image placeholders therefore followed
tool-call order, while media extraction still followed raw request order. When
tool results arrived out of order, images could be silently associated with
the wrong tool results.

DeepSeek V4.1 can also omit historical developer messages. Images from those
messages were still extracted even though no corresponding placeholder was
rendered, causing multimodal placeholder expansion to fail.

Make renderers report media source locations while emitting placeholders, and
use that order during multimodal extraction. DeepSeek V4.1 and Kimi K3 provide
their exact rendered media order. Renderers that preserve request order retain
the existing behavior through the default implementation.

Test Plan

  cargo nextest run --locked -p vllm-chat --lib \
    -E 'test(media_order_follows_reordered_tool_results) | test(media_order_excludes_dropped_historical_developer_content) |
    test(extract_media_parts_follows_renderer_order)'

  cargo nextest run --locked -p vllm-chat

  cargo nextest run --locked -p vllm-server --lib  -E 'test(non_stream_chat_image_url_reaches_engine_mm_features)'

  cargo fmt  --all -- --check

  cargo clippy --workspace --all-targets --all-features --locked -- -D warnings

Test Result

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[claude]

Claude Code Review

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[BugenZhao]

Thanks @reidliu41 this is a good catch! I've added a commit to put media_order into RenderedPrompt and use None for default ordering.

[BugenZhao]

/ci run

[github-actions]

✅ @reidliu41, CI is now available for this PR.

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[github-actions]

✅ Triggered Buildkite CI #88787 for commit 56440a17b610.