fix(runway): reject merged GitHub pulls listed after unmerged ones

[DivyanshGarg380]

Why?

validateStepIsGitHubStack contains an ineffective ordering check. It checks the prefix of pull requests returned by unmergedPulls, rather than validating the suffix containing unmerged pull requests.

Consequently, a merged pull request listed after an unmerged pull request can bypass validation and reach the GitHub stack lookup path.

What?

• Validate the suffix returned by unmergedPulls and reject any already-merged pull request found after the first unmerged pull request.
• Return merger.ErrInvalidRequest for invalid pull request ordering.
• Add regression tests covering valid ordering and merged pull requests appearing after unmerged ones.

Test Plan

• go test ./runway/extension/merger/github -run TestValidateStepIsGitHubStack_MergedMustPrecedeUnmerged -count=1
• go test ./runway/extension/merger/github -count=1
• python .\tool\bazel --output_user_root=C:\bazel test //runway/extension/merger/github:all

All tests passed.

Issue

Closes #835

[DivyanshGarg380]

Hi @behinddwalls, just following up on this PR. Would appreciate a review when you get a chance.
Thanks!