Improve oauth2 client "preferred username field" logic and the error handling (#30622)

Follow #30454
And fix #24957

When using "preferred_username", if no such field,
`extractUserNameFromOAuth2` (old `getUserName`) shouldn't return an
error. All other USERNAME options do not return such error.

And fine tune some logic and error messages, make code more stable and
more friendly to end users.

services/contexttest/context_tests.go

@@ -19,7 +19,9 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/cache"
 	"code.gitea.io/gitea/modules/gitrepo"
+	"code.gitea.io/gitea/modules/session"
 	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/translation"
 	"code.gitea.io/gitea/modules/web/middleware"
@@ -43,7 +45,8 @@ func mockRequest(t *testing.T, reqPath string) *http.Request {
 }
 
 type MockContextOption struct {
-	Render context.Render
+	Render       context.Render
+	SessionStore *session.MockStore
 }
 
 // MockContext mock context for unit tests
@@ -62,12 +65,17 @@ func MockContext(t *testing.T, reqPath string, opts ...MockContextOption) (*cont
 	base.Data = middleware.GetContextData(req.Context())
 	base.Locale = &translation.MockLocale{}
 
+	chiCtx := chi.NewRouteContext()
 	ctx := context.NewWebContext(base, opt.Render, nil)
 	ctx.AppendContextValue(context.WebContextKey, ctx)
+	ctx.AppendContextValue(chi.RouteCtxKey, chiCtx)
+	if opt.SessionStore != nil {
+		ctx.AppendContextValue(session.MockStoreContextKey, opt.SessionStore)
+		ctx.Session = opt.SessionStore
+	}
+	ctx.Cache = cache.GetCache()
 	ctx.PageData = map[string]any{}
 	ctx.Data["PageStartTime"] = time.Now()
-	chiCtx := chi.NewRouteContext()
-	ctx.Base.AppendContextValue(chi.RouteCtxKey, chiCtx)
 	return ctx, resp
 }