fix: use GTest Environment to finalize Arrow S3 after tests

Arrow's static destructor detects when S3 was initialized but not
finalized, causing a non-zero exit under sanitizers.  The previous
atexit approach caused segfaults because Arrow internals were already
partially destroyed.  Using a GTest Environment::TearDown() is the
correct solution: it runs after all tests complete but before static
destructors, making FinalizeS3() safe to call.

Also fix std::unexpected double-brace init for GCC 14 compatibility.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

src/iceberg/arrow/arrow_s3_file_io.cc

@@ -48,8 +48,8 @@ Status EnsureS3Initialized() {
     init_status = ::arrow::fs::InitializeS3(options);
   });
   if (!init_status.ok()) {
-    return std::unexpected<Error>{{.kind = ::iceberg::arrow::ToErrorKind(init_status),
-                                   .message = init_status.ToString()}};
+    return std::unexpected(Error{.kind = ::iceberg::arrow::ToErrorKind(init_status),
+                                .message = init_status.ToString()});
   }
   return {};
 #else

src/iceberg/test/arrow_s3_file_io_test.cc

@@ -18,15 +18,47 @@
  */
 
 #include <cstdlib>
+#include <iostream>
 #include <string>
 #include <unordered_map>
 
 #include <gtest/gtest.h>
 
+#ifdef ICEBERG_S3_ENABLED
+#  include <arrow/filesystem/s3fs.h>
+#endif
+
 #include "iceberg/arrow/arrow_file_io.h"
 #include "iceberg/arrow/s3_properties.h"
 #include "iceberg/test/matchers.h"
 
+#ifdef ICEBERG_S3_ENABLED
+namespace {
+
+/// \brief GTest environment that finalizes Arrow S3 after all tests complete.
+///
+/// Arrow's S3 initialization creates global state that must be cleaned up via
+/// FinalizeS3() before the process exits.  Without this, Arrow's static destructor
+/// detects the missing finalization and causes a non-zero exit (which fails under
+/// sanitizers).  GTest Environment::TearDown() runs after all tests but before
+/// static destructors, making it the safe place to finalize.
+class ArrowS3TestEnvironment : public ::testing::Environment {
+ public:
+  void TearDown() override {
+    auto status = ::arrow::fs::FinalizeS3();
+    if (!status.ok()) {
+      std::cerr << "Warning: FinalizeS3 failed: " << status.ToString() << std::endl;
+    }
+  }
+};
+
+// Register before main() runs.  GTest takes ownership of the pointer.
+[[maybe_unused]] auto* const kS3Env =
+    ::testing::AddGlobalTestEnvironment(new ArrowS3TestEnvironment);
+
+}  // namespace
+#endif
+
 namespace iceberg::arrow {
 
 TEST(ArrowS3FileIOTest, RejectsNonS3Uri) {