gh-108172: do not override OS preferred browser if it is a super-string of a known browser (GH-113011)

When checking if the registering browser is the "OS preferred browser", do not use a substring search - that makes no sense: one can have a preferred browser that looks like a super-string of a known browser, e.g. "firefox-nightly" vs "firefox".

https://github.com/python/cpython/issues/108172 explains in more detail, and lays out a potential better future enhancement for this case of just using xdg-open.  We'll go with this for now.

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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>