We always throw if `id_token_hint` is missing when `post_logout_redirect_uri` is used

The spec RP-Initiated Logout https://openid.net/specs/openid-connect-rpinitiated-1_0.html says:

3. Redirection to RP After Logout
   In some cases, the RP will request that the End-User's User Agent to be redirected back to the RP after a logout has been performed. Post-logout redirection is only done when the logout is RP-initiated, in which case the redirection target is the post_logout_redirect_uri parameter value sent by the initiating RP. An id_token_hint carring an ID Token for the RP is also RECOMMENDED when requesting post-logout redirection; if it is not supplied with post_logout_redirect_uri, the OP MUST NOT perform post-logout redirection unless the OP has other means of confirming the legitimacy of the post-logout redirection target. The OP also MUST NOT perform post-logout redirection if the post_logout_redirect_uri value supplied does not exactly match one of the previously registered post_logout_redirect_uris values. The post-logout redirection is performed after the OP has finished notifying the RPs that logged in with the OP for that End-User that they are to log out the End-User.

Currently we always throw, that is, stop the execution when id_token_hint is missing when post_logout_redirect_uri is used: https://github.com/simplesamlphp/simplesamlphp-module-oidc/blob/4c254afa8a80eb869351bc6dc161479f1f4d54a2/src/Server/RequestRules/Rules/PostLogoutRedirectUriRule.php#L59

Reading the spec, since id_token_hint is RECOMMENDED, we should not stop the execution, but simply not redirect to the post_logout_redirect_uri. We could show our own "you are logged out" page instead.

• TODO mivanci check how conformance tests handle this situation

src/Server/RequestRules/Rules/PostLogoutRedirectUriRule.php のリンクされたルールから始め、ログアウト後のリダイレクトに関する RP-Initiated Logout 仕様のセクションを読みます。TODO に記載されているとおり、conformance tests が id_token_hint の欠落をどのように処理するか確認します。完了の条件は、hint の欠落だけを理由にリクエストが停止されず、仕様で許可されていない限りログアウト後のリダイレクトが行われないことです。