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_test.cpp

@@ -42,6 +42,10 @@ static void test_defaults() {
     check(r.options.family.empty(), "family defaults to empty");
     check(r.options.task.empty(), "task defaults to empty");
     check(r.options.backend == "cpu", "backend defaults to cpu");
+    // backend_set separates the "cpu" default from an explicit backend:cpu —
+    // grpc-server merges the AUDIOCPP_DEFAULT_BACKEND fallback only when the
+    // model's options chose nothing.
+    check(!r.options.backend_set, "backend_set defaults to false");
     check(r.options.device == 0, "device defaults to 0");
     check(r.options.threads == 0, "threads defaults to 0");
     check(r.options.busy_timeout_ms == 0, "busy_timeout_ms defaults to 0");
@@ -68,6 +72,7 @@ static void test_scalar_options() {
     check(r.options.family == "qwen3_tts", "family parsed");
     check(r.options.task == "tts", "task parsed");
     check(r.options.backend == "cuda", "backend parsed");
+    check(r.options.backend_set, "backend_set records the explicit option");
     check(r.options.device == 1, "device parsed");
     check(r.options.threads == 8, "threads parsed");
     check(r.options.busy_timeout_ms == 30000, "busy_timeout_ms parsed");