Add unit and integration tests for Backstage plugin

[talissoncosta]

Summary

Add comprehensive test coverage for the Flagsmith Backstage plugin.

Parent Epic: #5641

Tasks

• Unit tests for FlagsmithClient.ts
• Unit tests for FlagsTab.tsx
• Unit tests for FlagsmithOverviewCard.tsx
• Unit tests for FlagsmithUsageCard.tsx
• Integration tests with mock API responses
• Set up test coverage reporting

Test Stack

Already in devDependencies:

• @testing-library/react ^14.0.0
• @testing-library/jest-dom ^6.0.0
• msw ^1.0.0 (for API mocking)