fix: determine mergeability of GitHub workflows from check suite

[henriklundstrom]

what

Use the conclusion of the check suite instead of the conclusion of the individual check run when determining if a pull request with a required workflow is mergeable when the gh-allow-mergeable-bypass-apply-flag is enabled.

• Use check suite conclusion rather than check run conclusion to determine required workflow outcome

why

• Resolves the issue Mergeability may be determined wrongfully on required workflows with multiple checks #5884
• The conclusion of an individual check run is insufficient for determining the conclusion of a workflow as it may have multiple check runs, the outcomes of which may differ, meaning a successful check run does not necessarily entail a successful workflow. Use the conclusion of the check suite instead, which holds the combined conclusion of each associated check run.

tests

• Adding a test case where a required workflow has multiple checks in the same suite but only the first is successful
  • This test fails with the implementation on main, but passes with the changes made by this PR
• Making a release on this feature branch: https://github.com/nordnet/atlantis/releases/tag/v0.37.0-pre.mergeability-from-check-suite-20250929-001
  • See the associated Docker image: ghcr.io/nordnet/atlantis:v0.37.0-pre.mergeability-from-check-suite-20250929-001-alpine@sha256:d7153cc2916d9c9bc0c6743ad1732bdea8d7eca73a1cd944f9f959695397cde5

references

[lukemassa]

Looks good to me!

[nvanheuverzwijn]

I just went extensively through this part of the code (for another unrelated bug) and this PR looks good to me.

[henriklundstrom]

Looks good to me!

@lukemassa is there anything more I should do in order to get this merged?