refactor: conditionally log missing credentials based on policy instead of always logging a warning

[ankitranjan7]

Description

Adjust proxy logging so missing credentials only log as a warning when the proxy is denying the request. In allow mode, the proxy now logs an informational passthrough message instead.

Motivation and Context

Issue #293 reports that when GitHub credentials are not present, Authsome logs the situation too harshly even though passthrough is intentional. This change makes the log level match the actual behavior: info for allowed passthrough, warning for denied requests.

Fixes #293.

How Has This Been Tested?

Ran:

• uv run pytest

Also added a focused regression test in tests/proxy/test_proxy.py for the no-credentials passthrough path.

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)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.