add existing only mode

[lucemia]

No description provided.

[Copilot]

PR Overview

This PR adds a new "existing only mode" that ensures only functions with an existing docstring are updated with exception documentation. Key changes include:

• Updating test cases to validate that functions without a docstring remain unchanged.
• Modifying process_directory and CLI argument parsing in cli.py to support a new only_existing flag.
• Enhancing docstring_utils.py to conditionally update docstrings based on the presence of an existing docstring.

Reviewed Changes

File	Description
src/auto_detect_exceptions/tests/test_cli.py	Adds a new test to verify that only functions with an existing docstring are updated.
src/auto_detect_exceptions/tests/test_docstring_utils.py	Modifies the test to assert that functions lacking a docstring are not updated.
src/auto_detect_exceptions/cli.py	Updates process_directory signature and CLI argument parsing to incorporate the only_existing flag.
src/auto_detect_exceptions/docstring_utils.py	Extends DocstringUpdater and update_function_docstrings to conditionally update functions.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.