fix: show PR head branch instead of base branch in Slack notifications

[kiwibel]

Summary

• Changed Slack notification to display the PR's head (source) branch instead of the base (target) branch
• Updated test fixture to include HeadBranch value
• Added internal unit test asserting the Branch attachment field uses HeadBranch

Why

The base branch is almost always main, which provides no useful context in Slack notifications. Showing the head branch instead makes it immediately clear which feature/fix was applied.

Test plan

• Unit tests pass (go test ./server/events/webhooks/...)
• Added TestCreateAttachments_HeadBranch to validate the Branch field value

[Copilot]

Pull request overview

Updates Atlantis’ Slack apply notifications so the displayed branch corresponds to the PR’s head/source branch, aligning the message with what users typically care about when reviewing applies.

Changes:

• Slack attachment “Branch” field now uses applyResult.Pull.HeadBranch instead of BaseBranch.
• Updated Slack client test setup fixture to populate HeadBranch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
server/events/webhooks/slack_client.go	Changes the Slack attachment branch value to the PR head branch.
server/events/webhooks/slack_client_test.go	Adds HeadBranch to the ApplyResult test fixture data.