fix: Replace surrogate characters before rendering

[SwiftyJunnos]

This PR resolves a crash issue in base.html rendering when a git commit message includes surrogate characters, such as emojis (e.g., 🐛).
The root cause was traced to the json_dumps_ensure_ascii function, which does not properly handle surrogate pairs, leading to a UnicodeEncodeError during template rendering.

[2025-06-30 00:32:51,740] [django.request::log_response::253] [ERROR] Internal Server Error: /labelstudio/
Traceback (most recent call last):
  File "/label-studio/.venv/lib/python3.12/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/django/views.py", line 90, in sentry_wrapped_callback
    return callback(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/label_studio/core/views.py", line 57, in main
    return render(request, 'home/home.html')
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/sentry_sdk/utils.py", line 1796, in runner
    return sentry_patched_function(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/sentry_sdk/integrations/django/templates.py", line 105, in render
    return real_render(request, template_name, context, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/django/shortcuts.py", line 26, in render
    return HttpResponse(content, content_type, status)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/django/http/response.py", line 376, in __init__
    self.content = content
    ^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/django/http/response.py", line 408, in content
    content = self.make_bytes(value)
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/label-studio/.venv/lib/python3.12/site-packages/django/http/response.py", line 317, in make_bytes
    return bytes(value.encode(self.charset))
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'utf-8' codec can't encode characters in position 3038-3039: surrogates not allowed

So I added a preprocessing step to replace or safely handle surrogate characters before they reach the JSON encoder, ensuring that the response can be encoded in UTF-8 without failure.

While there's currently no use of emojis in commit messages, this fix can prevent potential future errors in case such characters comes up someday.

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