This commit introduces a `isNotEqualTo(String)` assertion in order to
avoid false negatives when using the default Object-based assertion.
This is a risk since MediaTypeAssert has a `isEqualTo(String)` method
that overrides the base object method and parses the provided String
into a MediaType. Users may thus be tempted to use the reverse assertion
and expect the same parsing behaviour.
This commit also adds tests around the String parsing and the isNotEqual
cases.
Closes gh-32756
531da015e1