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

tests/nixpkgs_probe_parse.cpp

@@ -0,0 +1,55 @@
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.resolver;
+import cargoxx.util;
+import std;
+
+using cargoxx::resolver::parse_nix_eval_json;
+using cargoxx::util::ErrorCode;
+
+TEST_CASE("parse_nix_eval_json extracts version and outPath", "[resolver][nixpkgs]") {
+    constexpr std::string_view input = R"({
+        "version": "3.7.0",
+        "path": "/nix/store/abc-simdjson-3.7.0"
+    })";
+
+    auto r = parse_nix_eval_json("simdjson", input);
+    REQUIRE(r.has_value());
+    REQUIRE(r->attr == "simdjson");
+    REQUIRE(r->version == "3.7.0");
+    REQUIRE(r->out_path == "/nix/store/abc-simdjson-3.7.0");
+}
+
+TEST_CASE("parse_nix_eval_json accepts an empty version field",
+          "[resolver][nixpkgs]") {
+    constexpr std::string_view input = R"({
+        "version": "",
+        "path": "/nix/store/xyz-foo"
+    })";
+
+    auto r = parse_nix_eval_json("foo", input);
+    REQUIRE(r.has_value());
+    REQUIRE(r->version.empty());
+    REQUIRE(r->out_path == "/nix/store/xyz-foo");
+}
+
+TEST_CASE("parse_nix_eval_json fails when outPath is missing",
+          "[resolver][nixpkgs]") {
+    constexpr std::string_view input = R"({"version": "1.0"})";
+    auto r = parse_nix_eval_json("foo", input);
+    REQUIRE_FALSE(r.has_value());
+    REQUIRE(r.error().code == ErrorCode::ResolutionNetworkError);
+}
+
+TEST_CASE("parse_nix_eval_json fails on garbage input", "[resolver][nixpkgs]") {
+    auto r = parse_nix_eval_json("foo", "not-json-at-all");
+    REQUIRE_FALSE(r.has_value());
+    REQUIRE(r.error().code == ErrorCode::ResolutionNetworkError);
+}
+
+TEST_CASE("parse_nix_eval_json fails on a non-object root",
+          "[resolver][nixpkgs]") {
+    auto r = parse_nix_eval_json("foo", "[]");
+    REQUIRE_FALSE(r.has_value());
+    REQUIRE(r.error().code == ErrorCode::ResolutionNetworkError);
+}