Add validation constraints for repository creation fields (#36671)

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.

templates/swagger/v1_json.tmpl

@@ -23780,7 +23780,7 @@
           "x-go-name": "Name"
         },
         "object_format_name": {
-          "description": "ObjectFormatName of the underlying git repository",
+          "description": "ObjectFormatName of the underlying git repository, empty string for default (sha1)",
           "type": "string",
           "enum": [
             "sha1",