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GH-39444: [C++][Parquet] Fix crash in Modular Encryption (#39623)

**Rationale for this change:**

This pull request addresses a critical issue (GH-39444) in the C++/Python components of Parquet, specifically a segmentation fault occurring when processing encrypted datasets over 2^15 rows. The fix involves modifications in `cpp/src/parquet/encryption/internal_file_decryptor.cc`, particularly in `InternalFileDecryptor::GetColumnDecryptor`. The caching of the `Decryptor` object was removed to resolve the multithreading issue causing the segmentation fault and encryption failures.

**What changes are included in this PR?**

- Removal of `Decryptor` object caching in `InternalFileDecryptor::GetColumnDecryptor`.
- Addition of two unit tests: `large_row_parquet_encrypt_test.cc` for C++ and an update to `test_dataset_encryption.py` with `test_large_row_encryption_decryption` for Python.

**Are these changes tested?**

Yes, the unit tests (`large_row_parquet_encrypt_test.cc` and `test_large_row_encryption_decryption` in `test_dataset_encryption.py`) have been added to ensure the reliability and effectiveness of these changes.

**Are there any user-facing changes?**

No significant user-facing changes, but the update significantly improves the backend stability and reliability of Parquet file handling.  Calling DecryptionKeyRetriever::GetKey could be an expensive operation potentially involving network calls to key management servers. 

* Closes: #39444
* GitHub Issue: #39444

Lead-authored-by: Donald Tolley <tolleybot@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Adam Reeve <adreeve@gmail.com>
Co-authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>