Closes #34029

The crash is due to a stack overflow in our `html_to_markdown`
conversion; I've added a maximum depth of 200 for the recursion in that
crate to guard against this kind of thing.

Separately, we were treating all content-types other than `text/plain`
and `application/json` as HTML; I've changed this to only treat
`text/html` and `application/xhtml+xml` as HTML, and fall back to
plaintext. (In the original crash, the content-type was
`application/octet-stream`.)

Release Notes:

- agent: Fixed a potential crash when fetching large non-HTML files.