[Frontend][VLM] Add support for multiple multi-modal items in the OpenAI frontend

[ywang96]

This PR is an iteration of #7923 and in particular simplifies the multimodal placeholder padding logic.

Note that now we use a dictionary to keep track of unique placeholders and their counts, and the order of insertion is still preserved when we add them to the input prompt if they're missing. This is important for models that have specific ordered image tokens (e.g., <|image_1|><|image_2|>).

Co-authored-by: @petersalas

FIX #8053

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[ywang96]

I've added an additional test to cover too many images across messages, so this PR should be good to go.

[DarkLight1337]

LGTM