Migrate from os.path to pathlib for file path handling

[zliang-akamai]

📝 Description

Migrate low-level OS APIs to pathlib for handling file paths.

✔️ How to Test

pytest ./test/unit/linode_client_test.py -k test_ssh_key_create

pytest test/unit/linode_client_test.py -k test_instance_create

[Copilot]

Pull request overview

This PR migrates several file-path handling call sites from os.path APIs to pathlib.Path, aligning the SDK and docs with modern Python path handling.

Changes:

• Replaced os.path.isfile / os.path.expanduser checks with Path(...).expanduser().is_file() in key/cert loading helpers.
• Updated StackScript file detection to use Path.is_file().
• Updated Sphinx docs/conf.py to use pathlib when inserting the project root into sys.path.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
linode_api4/objects/nodebalancer.py	Uses Path(...).expanduser().is_file() for SSL cert/key file loading.
linode_api4/groups/profile.py	Uses Path(...).expanduser().is_file() for SSH public key loading from a file.
linode_api4/groups/linode.py	Uses Path.is_file() for StackScript “script is a file path” detection.
linode_api4/common.py	Uses Path(...).expanduser().is_file() for authorized_keys file loading.
docs/conf.py	Switches sys.path insertion to pathlib and updates the inline guidance comment.

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