Add success and failure handlers to code grant filter

[prasanna164-code]

Adds AuthenticationSuccessHandler and AuthenticationFailureHandler support to OAuth2AuthorizationCodeGrantFilter, following the structure of OAuth2AuthorizationCodeGrantWebFilter, so that the outcome of the Authorization Code grant can be customized, for example to forward into the application instead of redirecting.

Filter

• setAuthenticationSuccessHandler: invoked with the OAuth2AuthorizationCodeAuthenticationToken after the OAuth2AuthorizedClient has been saved. The default preserves the existing behaviour: redirect to the saved request if present, otherwise to the redirect_uri of the OAuth2AuthorizationRequest. setRequestCache keeps its contract and feeds the default handler.
• setAuthenticationFailureHandler: invoked with an OAuth2AuthenticationException wrapping the OAuth2AuthorizationException from the AuthenticationManager. The default preserves the existing redirect to the redirect_uri with error, error_description and error_uri appended.
• RedirectStrategy and RequestCache remain private to the default handlers; no public API is removed or deprecated.

DSL and docs

• OAuth2ClientConfigurer.AuthorizationCodeGrantConfigurer and the Kotlin AuthorizationCodeGrantDsl expose authenticationSuccessHandler and authenticationFailureHandler; the XML namespace gains authentication-success-handler-ref and authentication-failure-handler-ref on authorization-code-grant, so all three configuration styles offer the same options for this element.
• New "Handling the Authorization Response" section in the servlet OAuth2 client "Authorization Grant Support" page, with Java, Kotlin and XML examples.

Tests

• OAuth2AuthorizationCodeGrantFilterTests: null checks for both setters, custom success handler receives the token and suppresses the redirect while the authorized client is still saved, ForwardAuthenticationSuccessHandler forwards, custom failure handler receives the wrapped exception with the original OAuth2Error.
• OAuth2ClientConfigurerTests, AuthorizationCodeGrantDslTests and OAuth2ClientBeanDefinitionParserTests: both handlers wired through the Java DSL, the Kotlin DSL and XML.
• Existing redirect and request-cache tests are unchanged.

Closes gh-11069