refactor: clean up form binding & validation (#38873)

Clarify the "validation" and "error display" logic.

All the copied&pasted `Validate` functions are removed.

modules/structs/form.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+import (
+	"net/http"
+
+	"gitea.dev/modules/reqctx"
+	"gitea.dev/modules/translation/i18n"
+
+	"gitea.com/go-chi/binding"
+)
+
+// ValidateContext is a special context for form validation middleware
+type ValidateContext struct {
+	Locale i18n.LocaleTranslation
+	Data   reqctx.ContextData
+	Req    *http.Request
+	Resp   http.ResponseWriter
+}
+
+type FormDefaultValidator struct{}
+
+func (FormDefaultValidator) Validate(ctx *ValidateContext, errs binding.Errors) binding.Errors {
+	// this default validator only needs to return the errs as is because the "binding" function has already validated
+	return errs
+}