fix(oauth2): allow users to approve scope changes (#38942) (#39521)

Backport #38942 by @Harsh-128

Lets users approve an OAuth2 scope change on an existing grant instead
of failing with `a grant exists with different scope`. Fixes
https://github.com/go-gitea/gitea/issues/38940.

- Approving a different scope updates the existing grant. Issued tokens
follow immediately, since their scope is read from the grant.
- Confidential and trusted apps show the consent page when the scope set
changes, instead of silently reusing the old grant.
- An omitted `scope` reuses the existing grant's scope, like GitHub.
- The consent page lists newly added scopes.

<img width="500" alt="consent page with new scopes"
src="https://github.com/user-attachments/assets/40282353-32fe-4d87-9929-08aabbab32f1"
/>

Co-authored-by: Harsh Sharma <harshee2000@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

models/auth/oauth2.go

@@ -564,6 +564,12 @@ func (grant *OAuth2Grant) SetNonce(ctx context.Context, nonce string) error {
 	return nil
 }
 
+func UpdateGrantScope(ctx context.Context, grant *OAuth2Grant, newScope string) error {
+	grant.Scope = newScope
+	_, err := db.GetEngine(ctx).ID(grant.ID).Cols("scope").Update(grant)
+	return err
+}
+
 // GetOAuth2GrantByID returns the grant with the given ID
 func GetOAuth2GrantByID(ctx context.Context, id int64) (grant *OAuth2Grant, err error) {
 	grant = new(OAuth2Grant)