Fix taxa2tree: send link as a boolean, request the HTML route

[bmesuere]

umgap taxa2tree returns Invalid request body for every input. It has been completely broken against the live API.

Cause. The request sends args.url.to_string() for link, which puts the string "false" in the JSON where the API expects a boolean. The API deserializes the body through an untagged enum, so the type mismatch makes both variants fail to match and the whole request is rejected:

{"counts":{"562":2},"link":"false"}   ->  Invalid request body
{"counts":{"562":2},"link":false}     ->  {"id":1,"name":"Organism",...}

Second problem. With that fixed the command returned JSON, while its own docs say umgap taxa2tree < input.txt > output.html. The API serves the interactive page from a separate .html route (routes.rs:108), so this now requests that one unless --url was given.

Also switched to https; the endpoint was still being called over plain http.

This fixes scripts/umgap-visualize.sh -w too, which is built on this command — it was printing Invalid request body into the output file.

Still broken, but not ours

--url runs but does not produce a usable link. unipept-api returns a hardcoded placeholder for the link option:

if link {
    return TreeInformation::Link { gist: "test".to_string() };
}

so it answers {"gist":"test"} regardless of input. Worth an issue on that repo; nothing UMGAP can do about it.

Verification

Locally, with the toolchains CI uses: 38 tests pass, cargo +nightly fmt --check clean, and umgap taxa2tree now emits the HTML page for real input.

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