Having wantMessagesSigned=True globally means that it's also applied by
python3-saml to regular authentication SAMLResponses - making it require
the response to be signed, which is an issue because a feasible
alternative way that some IdPs (e.g. AzureAD) take by default is to sign
specifically the assertions in the SAMLResponse. This is also secure,
and thus we generally want to accept it.
Without this, the setting of wantMessagesSigned=True globally
in 4105ccdb17 causes a
regression for deployments that have already set up SAML with providers
such as AzureAD, making Zulip stop accepting the SAMLResponses.
Testing that this new logic works is handled by
test_saml_idp_initiated_logout_invalid_signature, which verifies that a
LogoutRequest without signature will be rejected.
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