fix: Fix base url regression to ensure trailing /

[stevehipwell]

Resolves #2978

This is a regression introduced in #2951.

---

Before the change?

• The GitHub client for the REST API can be configured with a base URL missing a trailing /
• The base URL needed to be configured exactly right

After the change?

• The GitHub client for the REST API base URL is configured correctly
• The base URL configuration is more flexible

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

---

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[stevehipwell]

@nickfloyd could you please review this PR?

[stevehipwell]

I'm refactoring this again to make it simpler.