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

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

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

modules/validation/refname_test.go

@@ -17,21 +17,18 @@ func Test_GitRefNameValidation(t *testing.T) {
 			data: TestForm{
 				BranchName: "test",
 			},
-			expectedErrors: binding.Errors{},
 		},
 		{
 			description: "Reference name contains single slash",
 			data: TestForm{
 				BranchName: "feature/test",
 			},
-			expectedErrors: binding.Errors{},
 		},
 		{
 			description: "Reference name has allowed special characters",
 			data: TestForm{
 				BranchName: "debian/1%1.6.0-2",
 			},
-			expectedErrors: binding.Errors{},
 		},
 		{
 			description: "Reference name contains backslash",