langchain_core.runnables.graph_mermaid.draw_mermaid_png calls this function, but the Mermaid API returns JPEG as default.
Add option `file_type` with default `png` type to be consistent.
Also background_color default was wrong and fixed with `!`

I mistakenly thought there was an error in the `background_color` option and added a `!` for correction. However, the `!` was already correctly added further down. This commit reverts the incorrect fix.

The `file_type` option was added as the second option, but the other function calling the function `_render_mermaid_using_api` without option names crashes.
The `file_type` option was added as the last option, so the effect is controlled locally.

I have updated the type annotations to be more stringent. `Optional[str]` to `Optional[Literal["jpeg", "png", "webp"]]`.

`Literal` has not been imported. Fixed.