feat(bulk-action): create reusable BulkActionToolbar component

[PARTH-TUSSLE]

Notes for Reviewers

This PR implements a standardized, reusable BulkActionToolbar custom component in Sistent (resolving sub-issue #1665 under the #1661 umbrella).

This component is designed to replace the duplicate and custom CustomToolbarSelect headers implemented across meshery-ui and other Layer5 projects.

Changes

• Created src/custom/BulkActionToolbar/BulkActionToolbar.tsx which implements the selection bar containing the selected count and an optional "Deselect ALL" checkbox action button.
• Reuses Sistent theme design tokens (theme.palette.background.secondary, theme.palette.background.card, theme.palette.text.default) for proper light/dark mode contrast.
• Returns null if selectedCount <= 0 to hide itself automatically.
• Created unit tests in src/__testing__/BulkActionToolbar.test.tsx to verify component mounting, children rendering, and click events.
• Registered the component in custom and root barrel files to prevent rollup-plugin-dts nested declaration bugs.

Integration and preview ->

When integrated with mui-datatables as a customToolbarSelect replacement, the BulkActionToolbar cleanly renders the deselect action [-], selection count (X selected), and custom action buttons (like Delete/Compare) next to the table's default selection label:

This PR fixes #1665

Signed commits

• [✅ ] Yes, I signed my commits.

[gemini-code-assist]

Code Review

This pull request introduces a new BulkActionToolbar component, including its unit tests and exports. The component displays a toolbar when items are selected, showing the count of selected items and an optional deselect button. The review feedback suggests avoiding hardcoded UI strings like 'Deselect ALL' and 'selected' to support internationalization (i18n). It recommends exposing these strings as configurable props with default values to maintain backward compatibility.

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[PARTH-TUSSLE]

Hey @KhushamBansal , would appreciate review on this whenever you get the time.