Form media given as html-safe strings rendered as paths on 6.1

[Adam Johnson]

Since #29490 (Django 4.1), Media assets may be given as hashable objects
implementing __html__(), rendered verbatim as the complete tag, per the old
​"Paths as objects" docs section.
Html-safe strings from mark_safe() satisfy that contract, but since
8096b5251090bf7539c59956e398b027c7525529 (#37085) they are treated as static
file paths instead: Media.__init__() now normalizes every
isinstance(path, str) entry into Script/Stylesheet, and SafeString is
a str subclass, so e.g.
forms.Media(js=[mark_safe('<script defer src="https://example.org/asset.js"></script>')])
renders as
<script src="/static/%3Cscript%20defer%20src%3D%22https%3A/..."></script>
rather than the tag verbatim as in 6.0. The same happens for css entries but
non-str @html_safe classes still work.

The fix is to skip normalization for any string that provides __html__(), so that they take the pre-existing verbatim rendering branch.