Refactor testing infrastructure to use test containers and fixtures

[Smartdevs17]

Context

Tests need isolated environments with proper fixture management.

Current Limitation

Tests share database and have no fixture management.

Expected Outcome

• Test containers for isolated environments
• Fixture factories for test data
• Snapshot testing for API responses
• Parallel test execution

Acceptance Criteria

• Docker test containers for database
• Fixture factories for all models
• API response snapshot testing
• Parallel test execution support
• Test data seeding scripts
• Coverage reporting with thresholds
• Flaky test detection and retry

Technical Scope

• backend/src/__tests__/ - Test files
• backend/src/__tests__/fixtures/ - Test fixtures
• backend/src/__tests__/helpers/ - Test utilities
• vitest.config.ts - Test configuration

[Muyideen-js]

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Hi! I'm a full-stack developer with TypeScript and Stellar ecosystem experience. I've reviewed this issue and can deliver a quality fix. Please assign this to me!

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[veloura-dev]

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Please maintainer..I can handle this professionally and make sure this issue is closed ASAP WITHOUT ERRORS.

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[Wilfred007]

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Can I work on this task?

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[drips-wave]

This issue has been completed by @Wilfred007 as part of the Stellar Wave Program's 7th Wave 🥳

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