fix: trigger PR event handling on PR updates for Gitea/Forgejo

[kfkonrad]

what

• Until now Gitea (and compatible forks) only triggers automatic terraform plans when the PR is first created or an issue is posted (or rather all other events are ignored by Atlantis).
• With this PR Atlantis will also run terraform plans when a PR on Gitea is updated (i.e. when a new commit is pushed/force-pushes).

why

• Gitea has different event types for PR creation (pull_request) and PR updated (pull_request_sync)
• Right now only the former is handled by Atlantis
• The pull_request and pull_request_sync events use the same schema and can thus be handled by the same case in the existing switch statement
  • Yes, I double checked that both in manual testing and the Gitea and Forgejo codebases

tests

• I tested my changes manually against my Forgejo instance
• I verified the schema compatibility regarding the way it's parsed in handleGiteaPullRequestEvent between pull_request and pull_request_sync in both the Gitea and Forgejo codebases and by manually comparing the payload of the respective events

references

none

[Copilot]

Pull request overview

This PR adds support for Gitea/Forgejo's pull_request_sync webhook event, which is triggered when a pull request is updated (e.g., new commits are pushed). Previously, Atlantis only responded to the initial pull_request event, meaning subsequent pushes to a PR would not trigger automatic Terraform plans. The fix is simple and follows established patterns used for other VCS providers in the codebase.

Changes:

• Added pull_request_sync to the case statement handling pull request events for Gitea/Forgejo, enabling Atlantis to run Terraform plans on PR updates