[blueberry808]

Purpose## Purpose

Adds schema-level support for the image_embeds chat content type to the experimental Rust frontend, as a claimed item on the Rust Frontend Feature Parity roadmap (#44280). image_embeds lets a client send pre-computed image embeddings instead of raw image bytes, bypassing the vision encoder — already supported by the Python frontend, gated behind --enable-mm-embeds.

This PR introduces the new ChatContentPart::ImageEmbeds variant and wires it through every place that exhaustively matches on ChatContentPart/Modality:

• request.rs: new ImageEmbeds { image_embeds: serde_json::Value, uuid: Option<String> } variant, is_multimodal().
• multimodal.rs: extract_media_parts() conversion to MediaContentPart::ImageEmbeds, media_part_limit_modality(), and a fix to prepare_multimodal()'s MmFeatureSpec.modality construction (was calling .to_string() against the wrong enum, a pre-existing bug surfaced once this crate compiled far enough) — now maps Modality::{Image,ImageEmbeds} to MmModality::Image on the wire.
• renderer/hf/mod.rs, renderer/inkling/mod.rs: embeds reuse the model's existing image_token placeholder, same as ImageUrl.
• renderer/kimi_k3/encoding.rs: embeds are rejected for now (Error::UnsupportedMultimodalContent), matching this renderer's existing behavior for video/audio.

This is a schema-only PR. Decoding the embedding payload itself (base64-encoded .npy or pickled torch.save tensor, per the Python ImageEmbeddingMediaIO), the --enable-mm-embeds CLI flag, and end-to-end request handling are follow-up work, tracked as separate PRs against this same feature.

Test Plan

cargo test across the workspace (vllm-chat, vllm-server, and dependents).

Test Result

All tests pass.

-Used Claude to help verify everything and with test creation

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @blueberry808.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @blueberry808.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork