start_tokens in synthetic prompt is not related to prompt_text

[tukwila]

Describe the bug
A clear and concise description of what the bug is.

When i do benchmark testing using synthetic prompts, the prompts are started with not unified language such as:

In this screenshot, start_tokens are different languages that are followed by english.

In this screenshot, start_tokens are different with following German.

Such prompt_tokens are inappropriate for benchmark testing.

and i write one script to dig into this problem:

import json

from guidellm.dataset.synthetic import (
    SyntheticDatasetCreator,
)

def test_handle_create_basic():
    # Example: German
    # data = "prompt_tokens=128,output_tokens=56, samples=10, source=https://www.gutenberg.org/cache/epub/30793/pg30793.txt" 
    # Example: English
    data = "prompt_tokens=128,output_tokens=56, samples=10, source=https://www.gutenberg.org/cache/epub/1342/pg1342.txt" 
    synthetic_creator=SyntheticDatasetCreator()
    res_list = []
    result = synthetic_creator.handle_create(
            data=data,
            data_args=None,
            processor="${local_path}/Qwen2.5-1.5B-Instruct",
            processor_args=None,
            random_seed=42,
        )
    res_list = result[:]['prompt']
    with open('./syntheic_prompts.json', 'w', encoding='utf-8') as f:
        json.dump(res_list, f, ensure_ascii=False, indent=4)
        
if __name__ == "__main__":
    test_handle_create_basic()

Expected behavior
A clear and concise description of what you expected to happen.

start word should be the same with the whole prompt.

Environment
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1. OS [e.g. Ubuntu 20.04]:
2. Python version [e.g. 3.12.2]: Python 3.9.9
3. guidellm version: 0.3.0

To Reproduce
Exact steps to reproduce the behavior:
step1. copy self-test scripts from above into local env.
step2. python ./test_synthetic_prompt.py

Errors
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Additional context
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