Branch rule details - feature test coverage

What does this MR do and why?

Adding extra coverage for Branch rules feature spec:

• when a predefined rule creation flow is started but no rule details provided
• defining project with a protected branch for a branch rule to have data to render Branch rule details page
• adding accessibility test case
• fixing accessibility violation of a low color contrast

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. spec/features/projects/settings/branch_rules_settings_spec.rb should pass without errors.

bundle exec rspec spec/features/projects/settings/branch_rules_settings_spec.rb

Related to #388129 (closed)