Fixed browser-sensitive admin CI tests

[9larsons]

Summary

• use the numeric keyCode path for the admin search shortcut test helper so keymaster sees the shortcut consistently across Chrome versions
• stub the billing iframe component's message wrapper instead of mutating cross-origin iframe contentWindow.postMessage
• avoid pipefail/SIGPIPE when selecting the first Docker image tag from multiline image tag input

Context

This came out of investigating failures on #28047 while testing Blacksmith runners. The changes are not Blacksmith-specific; they harden tests and CI shell behavior that can vary by browser/runtime.

Testing

• pnpm exec ember exam --split 1 --filter='Acceptance: Search'
• pnpm exec ember exam --split 1 --filter='Integration: Component: gh-billing-iframe'

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 5fd88a0c-3546-40eb-95e1-8e22de300c9b

📥 Commits

Reviewing files that changed from the base of the PR and between 6be7a48 and d43cad2.

📒 Files selected for processing (3)

• .github/actions/load-docker-image/action.yml
• ghost/admin/tests/acceptance/search-test.js
• ghost/admin/tests/integration/components/gh-billing-iframe-test.js

---

Walkthrough

This PR contains three independent, small-scale updates across infrastructure and test code. The Docker action refactoring simplifies tag extraction using shell parameter expansion instead of printf/head in two locations. The search test updates keyboard event dispatch to use numeric key codes. The billing iframe integration tests are refactored to stub an internal component method (_postMessageToBillingIframe) instead of the iframe contentWindow postMessage, reflecting changes to component internals.

Possibly related PRs

• TryGhost/Ghost#28025: The updated gh-billing-iframe-test.js stubs and asserts GhBillingIframe's internal _postMessageToBillingIframe method, which aligns with that PR's changes to route postMessage through that internal helper in the component implementation.
• TryGhost/Ghost#27875: Both PRs modify .github/actions/load-docker-image/action.yml in the same Docker tag-selection and image-pull flow, overlapping on the first inputs.image-tags extraction logic.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main focus of the changeset: fixing browser-sensitive tests in the admin CI.
Description check	✅ Passed	The description is directly related to the changeset and provides clear context about the three types of fixes being made and testing commands.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/browser-compat-ci-fixes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.82%. Comparing base (fcad5b8) to head (d43cad2).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #28070   +/-   ##
=======================================
  Coverage   73.81%   73.82%           
=======================================
  Files        1528     1528           
  Lines      129417   129417           
  Branches    15506    15509    +3     
=======================================
+ Hits        95527    95537   +10     
- Misses      32910    32924   +14     
+ Partials      980      956   -24     

Flag	Coverage Δ
admin-tests	54.03% <ø> (ø)
e2e-tests	73.82% <ø> (+<0.01%)	⬆️

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