fix(http): return 400 for empty/invalid JSON on required HTTP bodies

[AffanBinFaisal]

Description

Fixes #8762.

SageMaker single-model /invocations\ (and any path using \EVRequestToJson\ with a required JSON body) returned HTTP 500 for empty or non-JSON bodies because parse failures were reported as \TRITONSERVER_ERROR_INTERNAL.

Changes

• Return \TRITONSERVER_ERROR_INVALID_ARG\ for zero-length bodies when empty bodies are not allowed.
• Remap TritonJson parse errors (message prefix \ailed to parse the request JSON buffer) from \INTERNAL\ to \INVALID_ARG\ so \HttpCodeFromError\ returns 400.
• Add \L0_sagemaker\ regression tests (empty body, \\x00, whitespace); update \UNIT_TEST_COUNT.

Testing

• \python -m pre_commit run --files src/http_server.cc qa/L0_sagemaker/sagemaker_test.py qa/L0_sagemaker/test.sh\ (all hooks passed locally).
• Full \qa/L0_sagemaker/test.sh\ was not run here (requires NVIDIA CI model artifacts / /opt/tritonserver). Please rely on pipeline for end-to-end verification.

Contributor checklist

• CLA submitted to triton-cla@nvidia.com if not already on file (required before merge).

[AffanBinFaisal]

@yinggeh Kindly review my PR