fix: Respect cc_model_filenames in Python backend auto-fill

[nightflight-dk]

fix: Respect cc_model_filenames in Python backend auto-fill

Problem

When backend is set to "python" and default_model_filename is left empty, AutoCompleteBackendFields unconditionally sets default_model_filename to "model.py". This ignores any cc_model_filenames mappings the user may have configured to use a custom model filename (e.g., custom_model.py per device).

This causes the Python backend stub to always look for model.py, even when cc_model_filenames explicitly maps to a different file.

Fix

In model_config_utils.cc, the auto-fill of default_model_filename now checks that cc_model_filenames is also empty before defaulting to "model.py":

if (config->default_model_filename().empty() &&
    config->cc_model_filenames().empty()) {
  config->set_default_model_filename(kPythonFilename);
}

[nightflight-dk]

@whoisj this helps provide cc dependent implementation part of the same repo