refactor: move slack url creation to its own package

[santiagofn]

Part 2 of a series implementing threaded message support for the Slack notifier. Full picture: #5150. Depends on #5224.

Summary

Extracts Slack API URL resolution logic from slack.go into a new internal package notify/slack/internal/apiurl. Adds comprehensive unit tests covering all resolution paths (direct URL, file-based, global config).

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 403444e4-5d82-45b0-bd65-9080ddc9d982

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[SoloJacobs]

It looks like this was largely AI-generated. We'd ask that contributions be reviewed and understood by the author before submitting, including making sure the commit message accurately describes what the code actually does. The message here says "refactor" but the change alters behavior. Happy to look at it again once it's been properly reviewed on your end.