[Bugfix][V1] Merge multimodal embeddings by index instead of matching tokens

[DarkLight1337]

This PR fixes a mismatch in merging multi-modal embeddings when the model itself generates embedding placeholder tokens such as <image>. Although this error mainly occurs in V1, it can possibly occur in V0 as well. This PR focuses on the V1 case.

For V0 users, you can work around this by setting top_p so that the model has no chance of generating such tokens.

FIX #14897
FIX #15677
FIX #15764

[github-actions]

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