…d proxies

- Add undici ProxyAgent as dispatcher in custom fetch function
- Maintain backwards compatibility with existing agent option
- Add comprehensive unit tests for proxy functionality
- Fix proxy support for GitHub Enterprise Server environments

Fixes issue where @semantic-release/github cannot connect through
corporate proxies due to undici's fetch implementation not
respecting the agent option.

The solution provides both legacy agent support and new undici
ProxyAgent dispatcher for modern Octokit versions that use undici
as their fetch implementation.

gr2m approved these changes Nov 7, 2025