test: add unit and integration tests for Backstage plugin

[talissoncosta]

Summary

Add comprehensive unit and integration tests for the Flagsmith Backstage plugin, plus GitHub Actions CI for running tests on every PR.

Closes #6424

Changes

CI/CD

• Add test job to GitHub Actions workflow (runs yarn test --coverage --ci)

Test Infrastructure

• Add shared test fixtures with mock data (projects, environments, features, usage data)
• Add test utilities (renderWithBackstage, createMockFetchApi) for consistent test setup

Unit Tests

• Utilities (flagHelpers.ts): getFeatureEnvStatus, buildEnvStatusTooltip, calculateFeatureStats, paginate
• Theme (flagsmithTheme.ts): buildFlagUrl, buildProjectUrl URL builders
• API Client (FlagsmithClient.ts): All methods with mocked fetch and error handling
• Hooks: useFlagsmithProject, useFlagsmithUsage with loading/success/error states

Component Tests

• Shared components: LoadingState, MiniPagination, FlagStatusIndicator, FlagsmithLink, SearchInput
• Integration tests: FlagsTab, FlagsmithOverviewCard, FlagsmithUsageCard

Test Coverage

Category	Files	Tests
Utilities	2	~30
API Client	1	~25
Hooks	2	~20
Shared Components	5	~50
Integration	3	~40
Total	13	~165

Commits

1. ci: add test job to GitHub Actions workflow
2. test: add test fixtures and utilities
3. test(utils): add unit tests for flagHelpers
4. test(theme): add unit tests for flagsmithTheme
5. test(api): add unit tests for FlagsmithClient
6. test(hooks): add unit tests for custom hooks
7. test(components): add unit tests for shared components
8. test(components): add integration tests for main components

Test Plan

• All tests pass locally
• CI runs tests successfully on PR
• Test coverage meets project standards

🤖 Generated with Claude Code

[Zaimwa9]

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