🐛 `Template.Context` causes `InvalidOperationException` during JSON serialization

[gravity-api]

When returning Template objects from the API, the Context field (a Dictionary<string, object?>) sometimes triggers the following error during response serialization:

System.InvalidOperationException: Operation is not valid due to the current state of the object.
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(...)
   at System.Text.Json.Serialization.Converters.DictionaryDefaultConverter`3.OnWriteResume(...)

Impact:

• The API request fails with HTTP 500.
• Occurs when Context is modified while System.Text.Json is enumerating it during serialization.

Steps to Reproduce:

1. Return a Template object with a Context dictionary that may be mutated after the object is passed to the controller’s return Ok(...).
2. Trigger a concurrent update to Context during serialization.
3. Observe the InvalidOperationException in logs and the failed API response.

Expected Behavior:

• The API should return a successful JSON response, even if Context is being updated internally.
• The serializer should iterate over a stable snapshot of the dictionary.

Possible Root Cause:

Template.Context is a mutable dictionary reference that is being updated while ASP.NET Core’s System.Text.Json serializer is enumerating it. This causes the dictionary’s enumerator to throw an InvalidOperationException.

Additional Notes:

This is a concurrency safety issue and can be fixed without changing the public API by ensuring the returned object is detached from any live mutable state.