Add prep_sources MCP tool and Kerberize download_sources MCP tool

[antbob]

The rhpkg tool requires authentication even for basic operations like downloading sources and package prep. This commit adds a new prep_sources MCP tool and updates download_sources MCP tool to use Kerberos. If Kerberos auth is unavailable or fails the logic is to log a warning and continue so this should not change the existing behavior and should work for both rhpkg and centpkg tools.

[gemini-code-assist]

Code Review

This pull request introduces a new prep_sources MCP tool and adds Kerberos authentication support to the download_sources tool. The changes include refactoring command creation into a helper function and adding a helper to attempt Kerberos initialization while gracefully handling failures. The implementation looks good, and the new functionality is covered by unit tests. My main feedback is to improve error handling by capturing and reporting the stdout/stderr from subprocess calls, which will significantly aid in debugging failures.

_{Note: Security Review did not run due to the size of the PR.}

[TomasTomecek]

LGTM