fix(llama-cpp): preserve GPU layers during option passthrough (#11193)

Stage the negative GPU-layer sentinels expected by the upstream argument parser, then restore LocalAI resolved values unless a passthrough flag explicitly overrides them. This avoids the parser assertion that terminated the backend for any generic option.

Assisted-by: Codex:gpt-5

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

backend/cpp/llama-cpp/passthrough_options_test.cpp

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: MIT
+
+#include <cstdio>
+
+#include "passthrough_options.h"
+
+static int failures = 0;
+
+static void check(bool ok, const char * name) {
+    if (!ok) {
+        ++failures;
+        std::fprintf(stderr, "FAIL: %s\n", name);
+    }
+}
+
+static void test_stages_resolved_gpu_layers_for_upstream_parser() {
+    int main_gpu_layers = 99;
+    int draft_gpu_layers = 12;
+
+    const auto saved = llama_grpc::prepare_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers);
+
+    check(main_gpu_layers == -1, "main GPU layers use parser sentinel");
+    check(draft_gpu_layers == -1, "draft GPU layers use parser sentinel");
+
+    llama_grpc::restore_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers, saved);
+
+    check(main_gpu_layers == 99, "main GPU layers restored");
+    check(draft_gpu_layers == 12, "draft GPU layers restored");
+}
+
+static void test_keeps_explicit_passthrough_overrides() {
+    int main_gpu_layers = 99;
+    int draft_gpu_layers = 12;
+
+    const auto saved = llama_grpc::prepare_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers);
+
+    main_gpu_layers = 4;
+    draft_gpu_layers = 2;
+    llama_grpc::restore_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers, saved);
+
+    check(main_gpu_layers == 4, "main passthrough override retained");
+    check(draft_gpu_layers == 2, "draft passthrough override retained");
+}
+
+static void test_keeps_explicit_auto_passthrough_overrides() {
+    int main_gpu_layers = 99;
+    int draft_gpu_layers = 12;
+
+    const auto saved = llama_grpc::prepare_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers);
+
+    llama_grpc::restore_passthrough_gpu_layers(
+        main_gpu_layers, draft_gpu_layers, saved, true, true);
+
+    check(main_gpu_layers == -1, "main auto passthrough override retained");
+    check(draft_gpu_layers == -1, "draft auto passthrough override retained");
+}
+
+int main() {
+    test_stages_resolved_gpu_layers_for_upstream_parser();
+    test_keeps_explicit_passthrough_overrides();
+    test_keeps_explicit_auto_passthrough_overrides();
+    return failures == 0 ? 0 : 1;
+}