azure.ai.evaluation SDK cannot handle `conversation` input from a csv file.

- **Package Name**: azure.ai.evaluation
- **Package Version**: 1.8.0
- **Operating System**: Windows 11
- **Python Version**: 3.12.9

**Describe the bug**
In a csv file,

**To Reproduce**
Steps to reproduce the behavior:

pip install azure.ai.evaluation==1.8.0
Setup file `data.csv`:
```
conversation
"{""messages"": [{""role"": ""user"", ""content"": ""Hello""}, {""role"": ""assistant"", ""content"": ""Hello""}]}"
````
Setup file main.py
```python
from pathlib import Path
from azure.ai.evaluation import (
evaluate,
CoherenceEvaluator,
AzureOpenAIModelConfiguration,
)

dataset_path = Path("data.csv").resolve()

model_config = AzureOpenAIModelConfiguration(
azure_deployment="xxx",
azure_endpoint="xxx",
)

ret = evaluate(
data=dataset_path,
evaluators={"coherence": CoherenceEvaluator(model_config=model_config)},
evaluator_config={"default": {"conversation": "${data.conversation}"}},
)

```

**Expected behavior**
It should output coherence score

**Additional context**
In line 346 of the code https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/evaluation/azure-ai-evaluation/azure/ai/evaluation/_evaluators/_common/_base_eval.py

Looks like the input conversation is a string, instead of a Dict, the previous check `not in` doesn't take effect, because the stringified conversation json does contain the substring 'message'.