Implement get_checkpoint to allow content retrieval without restoration

[mariam851]

The Problem:
Currently, FileCheckpoints and AsyncFileCheckpoints do not implement the get_checkpoint method. This makes it impossible to retrieve the content of a specific checkpoint without performing a full restoration, which can lead to data loss of the current unsaved work.

The Solution:
This PR implements the get_checkpoint method in both synchronous and asynchronous classes.

Key Changes:
Direct File Access: Used internal _read_file and _read_notebook methods to read checkpoint data directly from the filesystem.

Path Handling: Implemented proper OS path to API path mapping to ensure compatibility across different operating systems, especially Windows.

Bypassing Hidden Directory Restrictions: By reading files directly, we bypass the ContentsManager 404 errors that occur when trying to access the hidden .ipynb_checkpoints directory via the standard API.

Testing:
I have verified the implementation using pytest and manual testing:

Synchronous test: Successfully retrieved versioned content of .txt files.

Asynchronous test: Successfully retrieved content using AsyncFileCheckpoints.

Windows Compatibility: Confirmed that path separators are handled correctly.

[minrk]

Thanks for the PR! Since you have tested this with pytest, can you make sure to include tests for these methods in the PR?