Handle failure to fetch Coveo credentials in UI

[lamATnginx]

Proposed changes

• Added error message on search failing to completely load if landing on the search page (e.g. docs.nginx.com/search.html).
• Added new partial called search-error.html for search error message.
  • Decoupled this from the search layout in case in the future, we might need to use it elsewhere if we change search providers. Also cleaner and consistent code in general like this.
• Added new test to check for missing credentials.
• Added new GH secret called COVEO_CREDENTIALS_BASE_URL.
• Refactored coveo testing code to move certain variables to be shared.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[github-actions]

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[github-actions]

❌ Playwright differences were detected.

View the Playwright report

[TasoOneAsia]

Looks like the playwright test is timing out in CI, I ran it again with the same result.

[lamATnginx]

Hold off on merging. We need to add some tickets around refactoring sidebar test + speed of downloading playwright browsers + dep.

Edit: Done

[TasoOneAsia]

Marking as draft per the last message.