Move TokenizedPrompt from Body to the request attribute store

[elevran]

What would you like to be added:

Move TokenizedPrompt from InferenceRequestBody.TokenizedPrompt to the per-request attribute store added in #1351. The existing token-producer plugin and its declared TokenizedPromptDataKey should be the writer; downstream consumers should read via scheduling.ReadRequestAttribute[*fwkrh.TokenizedPrompt](...).

Why is this needed:

TokenizedPrompt fits well into the per-request attribute store: it is request-scoped, produced once by a single plugin, consumed by several downstream plugins, and already has a DataKey declared in the producer DAG. Today the value lives on Body and not the top level requiest purely for historical reasons.
Moving it brings the producer/consumer wiring into agreement with how the framework expects request-scoped data to flow and removes a hidden mutation surface tat exists with having it on the Body.

[elevran]

CC @vMaroon @liu-cong

[ahg-g]

The only issue is how to align this with request protocols that include tokens (like the vllm's generate and some grpc apis)

[elevran]

The pre-tokenized input is copied into TokenizedPrompt. The proposal is to do the same only stored TokenizedPrompt as an attribute instead of a field on the request. Same format, same procissing, different storage location.

[github-actions]

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[yuanhuizang-cloud]

Hi @elevran , I noticed this issue has gone stale and is still assigned to you.
I'd be interested in working on it if you're not actively working on it.

I've checked current main and confirmed that TokenizedPrompt is still stored on InferenceRequestBody while the producer/consumer dependency is already declared through TokenizedPromptDataKey.

Would you be okay with me taking this on?

[yuanhuizang-cloud]

I went ahead and opened #2653 for this issue.

The PR moves TokenizedPrompt to the per-request attribute store and updates the affected producers/consumers, including the pre-tokenized vLLM gRPC path.

Happy to adjust the approach based on review feedback.