apacheGH-514: [Parquet] Infer schema when projection is null in ParquetReader

This change enables reading Parquet files without an explicit projection schema.
If `options.projection` is not provided, the reader now infers the Iceberg schema
from the Parquet file's Arrow schema using the Arrow C++ API.

*   Modified `src/iceberg/parquet/parquet_reader.cc`:
    *   Removed null check for `projection` in `Open`.
    *   Implemented `InferIcebergSchema` and `ConvertArrowType` to convert `arrow::Schema` to `iceberg::Schema` directly, avoiding complex C-ABI/nanoarrow dependencies.
    *   Used inferred schema when `projection` is null.
    *   Used `::arrow::` prefix to avoid namespace ambiguity.
*   Added `src/iceberg/test/parquet_reader_no_projection_test.cc` to verify the fix.
*   Updated `src/iceberg/test/CMakeLists.txt` to register the new test file.

Co-authored-by: wgtmac <4684607+wgtmac@users.noreply.github.com>

Author: google-labs-jules[bot]