phsl / new-api

Some upstream Kling deployments still expect the legacy `model` key
instead of `model_name`.
This change adds the `model` field to `requestPayload` and populates it
with the same value as `model_name`, ensuring the generated JSON works
with both old and new versions.

Changes:
• Added `Model string "json:\"model,omitempty\""` to `requestPayload`
• Set `Model` alongside `ModelName` in `convertToRequestPayload`
• Updated comments to clarify compatibility purpose

Result:
Kling task requests now contain both `model_name` and `model`, removing
integration issues with upstreams that only recognize one of the keys.