test: For the multi-provider cli

[kush-gupt]

A seperate PR on top of #33 that aims to add test coverage to the CLI.

The test file was created with assistance from Jules (Google Gemini 2.5 pro agent)

[mergify]

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[ceberam]

Thanks @kush-gupt for your contribution.
Regarding the AI-generated code in the test file, while we do not generally prohibit AI-generated code, we would require to:

• Ensure that the terms and conditions of the generative AI tool (in this case, Jules) (a) allow its AI Code to be used for external Open Source development and (b) are consistent with submitting the AI Code under the Docling community contribution guidelines, including enabling it to be used by the project under its Open Source license (MIT license)
• As a best practice, since you have have substantially altered the file using an AI tool, we recommend adding a comment in the file header (“// this file has been modified with the assistance of AI Tool [X v.Y]”).