Add new text followup preprocessor

[jeffbl]

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Resolves #918 and resolves #934.

This PR adds a new preprocessor text-followup designed to let a Monarch user speak a followup query for the current graphic, and receive an LLM-generated response. The new text-followup schema, which contains both a brief and a full response tag, was merged earlier.

It also implements the prompt override as described in #931, but that issue is being left open since this needs to also be implemented in the other LLM-based preprocessors before it is closed. The format for this override is in the README.md file.

Testing includes running Venissa's script (see below) repeatedly with different prompts, which found many ways llama3.2-vision resisted producing good JSON that followed the prompt. Venissa also tested it with the text-followup-handler she is implementing, and it works end-to-end. It still fails sometimes, but these failures seem to be infrequent with the current prompt.

jq '.followup.query = "Are there any animals?"' /home/venicq/follow-up.json | /var/docker/image/bin/sendimagereq /dev/stdin https://unicorn.cim.mcgill.ca/image/render/preprocess/ | jq '.preprocessors."ca.mcgill.a11y.image.preprocessor.text-followup"'

Also note that I did git merge from main into this branch, which means there are some spurious commits listed in this PR, but it should merge cleanly to main. Please let me know if there is any trouble, and I'll do rebase instead in the future, which I think would have avoided this problem.

@VenissaCarolQuadros not making you a formal reviewer, but since you are the main consumer of this preprocessor, figured you might want to take a look at how the sausage is made, and any comments or suggestions are welcome.

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