[M5+] add resolver::nixpkgs_probe (nix eval wrapper)

tests/nixpkgs_probe_live.cpp

@@ -0,0 +1,40 @@
+// Network/nix-eval-gated integration test for resolver::nixpkgs_probe.
+// Skipped unless CARGOXX_NETWORK_TESTS=1 is set in the environment.
+
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.resolver;
+import cargoxx.util;
+import std;
+
+namespace {
+
+auto network_tests_enabled() -> bool {
+    auto* env = std::getenv("CARGOXX_NETWORK_TESTS");
+    return env != nullptr && std::string_view{env} == "1";
+}
+
+} // namespace
+
+TEST_CASE("nixpkgs_probe finds 'hello'", "[resolver][network]") {
+    if (!network_tests_enabled()) {
+        SKIP("CARGOXX_NETWORK_TESTS != 1");
+    }
+    auto r = cargoxx::resolver::nixpkgs_probe("hello");
+    REQUIRE(r.has_value());
+    REQUIRE(r->attr == "hello");
+    REQUIRE_FALSE(r->out_path.empty());
+    REQUIRE(r->out_path.starts_with("/nix/store/"));
+    // GNU hello has a stable, conventional version field.
+    REQUIRE_FALSE(r->version.empty());
+}
+
+TEST_CASE("nixpkgs_probe rejects an unknown attribute", "[resolver][network]") {
+    if (!network_tests_enabled()) {
+        SKIP("CARGOXX_NETWORK_TESTS != 1");
+    }
+    auto r = cargoxx::resolver::nixpkgs_probe(
+        "definitely_not_a_real_pkg_cargoxx_xyzzy");
+    REQUIRE_FALSE(r.has_value());
+    REQUIRE(r.error().code == cargoxx::util::ErrorCode::ResolutionUnknownPackage);
+}