feat(audio-cpp): AUDIOCPP_DEFAULT_BACKEND fallback for models without a backend option (#12133)

Models whose options carry no explicit backend: open their session on the
CPU backend even in accelerator images. The gallery entries carry
backend:best since #11892; this covers hand-written model configurations
the same way, per deployment: the environment variable supplies the
fallback, an explicit backend: option always wins (merged beside the
existing threads and maingpu fallbacks), and validation reuses the
option parser.

Assisted-by: Claude:claude-fable-5

Signed-off-by: Plamen K. Kosseff <p.kosseff@gmail.com>

backend/cpp/audio-cpp/model_options.cpp

@@ -120,6 +120,7 @@ ParsedOptions parse_model_options(const std::vector<std::string> &entries) {
                 return parsed;
             }
             parsed.options.backend = value;
+            parsed.options.backend_set = true;
         } else if (key == "model_spec_override") {
             parsed.options.model_spec_override = value;
         } else if (key == "device") {