test(command): cover exec failure paths

Command tests did not assert behavior when exec fails in child processes.

I added deterministic regression coverage that forces execl/execlp failure and
verifies non-zero exit status propagation for both open() and forkExec paths.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

test/utils/command.cpp

@@ -0,0 +1,56 @@
+#if __has_include(<catch2/catch_test_macros.hpp>)
+#include <catch2/catch_test_macros.hpp>
+#else
+#include <catch2/catch.hpp>
+#endif
+
+#include <cerrno>
+#include <list>
+#include <mutex>
+#include <sys/wait.h>
+#include <unistd.h>
+
+std::mutex reap_mtx;
+std::list<pid_t> reap;
+
+extern "C" int waybar_test_execl(const char* path, const char* arg, ...);
+extern "C" int waybar_test_execlp(const char* file, const char* arg, ...);
+
+#define execl waybar_test_execl
+#define execlp waybar_test_execlp
+#include "util/command.hpp"
+#undef execl
+#undef execlp
+
+extern "C" int waybar_test_execl(const char* path, const char* arg, ...) {
+  (void)path;
+  (void)arg;
+  errno = ENOENT;
+  return -1;
+}
+
+extern "C" int waybar_test_execlp(const char* file, const char* arg, ...) {
+  (void)file;
+  (void)arg;
+  errno = ENOENT;
+  return -1;
+}
+
+TEST_CASE("command::execNoRead returns 127 when shell exec fails", "[util][command]") {
+  const auto result = waybar::util::command::execNoRead("echo should-not-run");
+  REQUIRE(result.exit_code == waybar::util::command::kExecFailureExitCode);
+  REQUIRE(result.out.empty());
+}
+
+TEST_CASE("command::forkExec child exits 127 when shell exec fails", "[util][command]") {
+  const auto pid = waybar::util::command::forkExec("echo should-not-run", "test-output");
+  REQUIRE(pid > 0);
+
+  int status = -1;
+  REQUIRE(waitpid(pid, &status, 0) == pid);
+  REQUIRE(WIFEXITED(status));
+  REQUIRE(WEXITSTATUS(status) == waybar::util::command::kExecFailureExitCode);
+
+  std::scoped_lock<std::mutex> lock(reap_mtx);
+  reap.remove(pid);
+}