Temporarily skip accessibility violations in Storybook stories by adding a11y skip rules

What does this MR do and why?

This MR addresses accessibility violations detected in the Storybook test pipeline by adding appropriate a11y skip rules to the affected stories.

Changes

Added parameters.a11y.config.rules to skip accessibility violations in the following stories:

• admin/broadcast_messages/base.stories.js: Skip empty-table-header in Default story
• vue_shared/import/import_source_list_table.stories.js: Skip label in Functional story
• packages_and_registries/virtual_registries/upstreams_list.stories.js: Skip role-img-alt in Default story
• packages_and_registries/virtual_registries/registry_upstream_item.stories.js: Skip role-img-alt in Default story
• members/promotion_requests/app.stories.js: Skip aria-required-children and color-contrast in Default, LoadingState, ErrorState, and ProjectLevelView stories
• vue_shared/expand_collapse_button/expand_collapse_button.stories.js: Skip aria-valid-attr-value in MultipleSections story
• analytics/analytics_dashboards/dashboard_list.stories.js: Skip aria-prohibited-attr and button-name in Default story
• insights/insights_chart.stories.js: Skip role-img-alt in NoData story

Rules Skipped

• empty-table-header: Table header text should not be empty
• label: Form elements must have labels
• role-img-alt: [role="img"] elements must have alternative text
• aria-required-children: Certain ARIA roles must contain particular children
• color-contrast: Elements must meet minimum color contrast ratio thresholds
• aria-valid-attr-value: ARIA attributes must conform to valid values
• aria-prohibited-attr: Elements must only use permitted ARIA attributes
• button-name: Buttons must have discernible text

These violations are component-specific and do not represent broader accessibility issues in the codebase.

References

• Make storybook-test job under test-frontend pip... (#562323)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check if test-storybook CI job has any accessibility violations. Should be none.
2. Each skipped rule should have a corresponding issue for fixing it with proper bug, accessibility severity and owning group lebels.

MR acceptance checklist

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