fix: export ThreadPool symbols and silence google-runtime-int lint

Two CI failures from the previous push:

1. Meson lanes (Linux/macOS/Windows) failed to link `util_test` against
   `libiceberg.so` with undefined references to `iceberg::ThreadPool::*`. The
   project sets CXX_VISIBILITY_PRESET=hidden, so symbols crossing the shared-
   library boundary need ICEBERG_EXPORT. CMake builds use static archives
   locally, which is why this only surfaced on the Meson shared-lib lanes.
   Adding ICEBERG_EXPORT to the class follows the same pattern as
   RetryRunner/ExpireSnapshots: the `_internal.h` naming is a header-org hint,
   not a visibility decision.

2. cpp-linter (clang-tidy google-runtime-int) flagged `std::atomic<long>` in
   thread_pool_test.cc. Switched to `std::atomic<std::int64_t>`.

The zizmor failure on the same run was a docker-pull timeout pulling
`ghcr.io/zizmorcore/zizmor:latest` — pure infra flake, no source change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: shangxinli

src/iceberg/test/thread_pool_test.cc

@@ -19,6 +19,7 @@
 
 #include <atomic>
 #include <chrono>
+#include <cstdint>
 #include <span>
 #include <stdexcept>
 #include <thread>
@@ -47,7 +48,7 @@ TEST(ThreadPoolTest, RunAndWaitProcessesEveryItem) {
   std::vector<int> items(100);
   for (int i = 0; i < 100; ++i) items[i] = i;
 
-  std::atomic<long> sum{0};
+  std::atomic<std::int64_t> sum{0};
   pool.RunAndWait<int>(std::span<const int>(items),
                        [&](int v) { sum.fetch_add(v, std::memory_order_relaxed); });
 

src/iceberg/util/thread_pool_internal.h

@@ -30,6 +30,8 @@
 #include <utility>
 #include <vector>
 
+#include "iceberg/iceberg_export.h"
+
 namespace iceberg {
 
 /// \brief Fixed-size worker pool for fire-and-wait task batches.
@@ -40,7 +42,7 @@ namespace iceberg {
 /// the constructor and joined in the destructor.
 ///
 /// All public methods are thread-safe.
-class ThreadPool {
+class ICEBERG_EXPORT ThreadPool {
  public:
   /// \brief Construct a pool with `num_workers` worker threads. Must be > 0.
   explicit ThreadPool(std::size_t num_workers);