fix(cli): handle missing [codecarbon] section and write api_endpoint via ConfigParser

[sohammishra864-wq]

Description

• get_config() now returns {} instead of None when the config file has no [codecarbon] section.
• get_api_endpoint() now sets the default api_endpoint through ConfigParser instead of raw-appending a line, so it always lands under [codecarbon].

Related Issue

Fixes #1432

Motivation and Context

show_config() crashed with TypeError on a config file that had no [codecarbon] section. get_api_endpoint() wrote the key under the last section of multi-section files, so it was never found and got appended again on every call.

How Has This Been Tested?

Added two tests in tests/cli/test_cli_utils.py. Both fail on master and pass with this change. pytest tests/cli: 76 passed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

AI Usage Disclosure

• 🟥 AI-vibecoded
• 🟠 AI-generated
• ⭐ AI-assisted
• ♻️ No AI used

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the docs/how-to/contributing.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.