Add validation constraints for repository creation fields (#36671) (#36757)

Backport #36671 by @lunny

Adds validation constraints to repository creation inputs, enforcing
max-length limits for labels/license/readme and enum validation for
trust model and object format. Updates both the API option struct and
the web form struct to keep validation consistent.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/forms/repo_form.go

@@ -27,9 +27,9 @@ type CreateRepoForm struct {
 	DefaultBranch string `binding:"GitRefName;MaxSize(100)"`
 	AutoInit      bool
 	Gitignores    string
-	IssueLabels   string
-	License       string
-	Readme        string
+	IssueLabels   string `binding:"MaxSize(255)"`
+	License       string `binding:"MaxSize(100)"`
+	Readme        string `binding:"MaxSize(255)"`
 	Template      bool
 
 	RepoTemplate    int64
@@ -41,7 +41,7 @@ type CreateRepoForm struct {
 	Labels          bool
 	ProtectedBranch bool
 
-	ForkSingleBranch string
+	ForkSingleBranch string `binding:"MaxSize(255)"`
 	ObjectFormatName string
 }