fix: usage of ReadDataFromJson in array tensors

[v-shobhit]

What does the PR do?

The generate and generate_stream endpoints did not seem to work when directly querying TRTLLM backend with input tokens. This is because the HTTPAPIServer::GenerateRequestClass::ExactMappingInput does not send the correct size of an array input to ReadDataFromJson.

This PR also fixes triton-inference-server/tensorrtllm_backend#369

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Test plan:

Added a new test case to L0_http job.
Internal CI pipeline id: 18800660

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• closes GitHub issue: #xxx