[BUG]: Add the nemo_retriever quickstart snippets to unit tests

[randerzander]

Version

26.3

Which installation method(s) does this occur on?

Source

Describe the bug.

As we iterate on rapidly on the library code, we need to make sure the nemo_retriever quickstart guide API example snippets keep working.

Minimum reproducible example

Each of these should be a unit test:

The [base snippet](https://github.com/nvidia/nemo-retriever/tree/main/nemo_retriever#ingest-a-test-pdf)

[markdown formatting](https://github.com/nvidia/nemo-retriever/tree/main/nemo_retriever#inspect-extracts)

the [retrieval example](https://github.com/nvidia/nemo-retriever/tree/main/nemo_retriever#run-a-recall-query)

[ingesting docx/pptx/.txt/image/audio/video files](https://github.com/nvidia/nemo-retriever/tree/main/nemo_retriever#ingest-other-types-of-content)

and

[remote inference w/ build hosted models](https://github.com/nvidia/nemo-retriever/tree/main/nemo_retriever#run-with-remote-inference-no-local-gpu-required)

Relevant log output

Other/Misc.

No response