build: register thread_pool_internal sources in meson build

CMake builds picked up the new files automatically via the explicit source
list, but the parallel meson build needs the same registration. Adds
util/thread_pool_internal.cc to src/iceberg/meson.build and thread_pool_test.cc
to src/iceberg/test/meson.build (under util_test).

Fixes the link error on the Meson CI lanes:
  undefined reference to `iceberg::ThreadPool::ThreadPool(unsigned long)'

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

Author: shangxinli

src/iceberg/meson.build

@@ -132,6 +132,7 @@ iceberg_sources = files(
     'util/string_util.cc',
     'util/struct_like_set.cc',
     'util/temporal_util.cc',
+    'util/thread_pool_internal.cc',
     'util/timepoint.cc',
     'util/transform_util.cc',
     'util/truncate_util.cc',

src/iceberg/test/meson.build

@@ -96,6 +96,7 @@ iceberg_tests = {
             'roaring_position_bitmap_test.cc',
             'string_util_test.cc',
             'struct_like_set_test.cc',
+            'thread_pool_test.cc',
             'transform_util_test.cc',
             'truncate_util_test.cc',
             'url_encoder_test.cc',