Clarify the input handling of session template creation API

[achimnol]

In lablup/backend.ai-manager#480, #1406, and #1668, the session template creation API became able to process only the yaml.safe_load_all()'s return value, which is a generator.

It may crash with the json.loads() output as it would just return a dict, when the client submits a JSON data instead of YAML data. This is a potential bug.

Along with #3183, we need to clarify whether the API handler decomposes the input parameter object and applies individual validation to its parts or delegate the entire parameter interpretation to a structured schema (e.g., Pydantic).

[achimnol]

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