chore(deps): bump cypress/included from 14.5.3 to 15.9.0 in /.buildkite

Bumps cypress/included from 14.5.3 to 15.9.0.

---
updated-dependencies:
- dependency-name: cypress/included
  dependency-version: 15.9.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.buildkite/Dockerfile

@@ -1,5 +1,5 @@
 FROM ruby:4.0.1-slim-bookworm AS ruby
-FROM cypress/included:14.5.3 AS cypress
+FROM cypress/included:15.9.0 AS cypress
 FROM python:3.14.2-bookworm AS python
 
 FROM golang:1.25.6-bookworm AS golang