Fix inconsistent OAuth2 login redirect URI action in MVC stack

[chanbinme]

This PR fixes an inconsistency in the Spring Security OAuth2 client where the MVC stack generates an incorrect OAuth2 login redirect URI due to using "authorize" as the default action, unlike the Reactive stack which uses "login".

Closes: gh-16941

Problem

• The existing MVC implementation (DefaultOAuth2AuthorizationRequestResolver) sets the redirect URI action to "authorize", resulting in URIs like /authorize/oauth2/code/{registrationId}.
• The Reactive implementation uses "login", producing the correct URI /login/oauth2/code/{registrationId}.

Changes

• Modified resolve(HttpServletRequest, String) method in MVC resolver to always use "login" as the redirect URI action.
• Removed redundant client registration and grant type validation here, assuming they are handled downstream.

Before:

@Override
public OAuth2AuthorizationRequest resolve(HttpServletRequest request, String registrationId) {
    if (registrationId == null) {
       return null;
    }
    String redirectUriAction = getAction(request, "authorize");
    return resolve(request, registrationId, redirectUriAction);
}

After:

@Override
public OAuth2AuthorizationRequest resolve(HttpServletRequest request, String registrationId) {
    if (registrationId == null) {
       return null;
    }
    String redirectUriAction = getAction(request, "login");
    return resolve(request, registrationId, redirectUriAction);
}

Impact & Benefits

• Ensures uniform OAuth2 login redirect URI format across MVC and Reactive stacks.

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Please review and provide feedback. Thank you.