Standardize mask validation error

[evaline-ju]

Describe the bug

Currently while most validation errors return 422, failing masks validation on the classification endpoints returns 400 with an error like

{"code":400,"details":"Failed to parse the request body as JSON: guardrail_config.input.masks[0]: trailing characters at line 5 column 44"}%

Sample Code

Example call

curl -v -H "Content-Type: application/json" -k --request POST \
--data '{"inputs": "There once was...",
    "model_id": "llm-model-id",
    "guardrail_config": {
        "input": {
            "models": {}, "masks": [[0, 6, 7]]
        },
        "output": {
            "models": {"detector-id": {}}
        }
    }
}' \
    http://localhost:8033/api/v1/task/server-streaming-classification-with-text-generation

Expected behavior

422/same code as validation errors. In case all request validation is updated from 400 to 422 we still want the error message to be informative about masks format (only start, end) rather than referring to "trailing characters" which is less intuitive

Error can look like the case of "masks": [[0, 6]] which returns
{"code":422,"details":"invalid masks"}

Observed behavior

{"code":400,"details":"Failed to parse the request body as JSON...