Scope status webhooks to their repository

[gulkaran]

TL;DR

• Status webhooks currently update every tracked commit with the same SHA, including commits in unrelated repositories. Restrict updates to the payload repository while preserving updates across its stacks. The regression fails before the fix; afterward, all 19 webhook controller tests (52 assertions) and lint for both changed files pass.

What changed?

• Scope StatusHandler commit selection to the repository's stack IDs using stack_id: stacks.select(:id).
• Give WebhooksControllerTest payloads the correct repository full name and consistent JSON keys.
• Add one regression covering matching production and canary commits alongside a same-SHA commit in another repository under the same organization.

How to test?

1. Run the targeted tests and lint checks.

bundle exec ruby -Itest test/controllers/webhooks_controller_test.rb
bundle exec rubocop app/models/shipit/webhooks/handlers/status_handler.rb test/controllers/webhooks_controller_test.rb

Why make this change?

• CI results belong to a repository even when its commits also exist elsewhere. Applying another repository's result can incorrectly influence Shipit's deployment decisions.