Only use valid pastKeyValues when sequence length is greater than 0

[LeanBitLab]

• Updated ProofreadService to properly validate the sequence length of pastKeyValues tensors before trying to pass the cached state to ONNX Runtime.
• Only sets input tokens and caches when isValidPkv correctly identifies valid sequence length.
• Updated fallback logic to generate zero tensors for any step if the cache evaluates to invalid.

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PR created automatically by Jules for task 6163460110815717061 started by @LeanBitLab

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