test(manager/mise): don't allow unknown backend types

[jamietanna]

Changes

As noted in #46343, upstream mise renamed a large number of pipx
registry entries to use the pypi backend instead, e.g. ansible
went from pipx:ansible to pypi:ansible.

This was caught during code review, but we should add something to
automagically catch this, by making sure that none of the backends in
use on an updated mise registry is unknown to Renovate.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
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AI assistance disclosure

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• Yes — other (please describe):

Claude Sonnet 5 (via Claude Code) drafted the test and this PR description.

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