[TESTING & QA] Create mock data fixtures for all test suites

[dev-fatima-24]

Priority: high

Description

Each test suite currently creates its own ad-hoc test data, leading to inconsistency and duplication. A shared fixtures directory must be created with factory functions for all domain objects: projects, credit batches, listings, retirements, and certificates. This reduces test maintenance overhead.

Acceptance Criteria

• Factory functions created for: Project, CreditBatch, Listing, Retirement, Certificate
• Each factory accepts optional overrides for specific fields
• Fixtures are used consistently across all backend integration tests
• Fixtures are documented with the shape of each object
• Fixtures are reset between test runs using database transactions

[Venrable18]

@Venrable18 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Good day maintainer..

I’m a full-stack and test integration developer…. 

Please assign this task to me…

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[Timothy2025-20]

@Timothy2025-20 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi team, am a software engineer with her of experience and I would love to address and resolve this issue if assigned to me. Thank you 🙏

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This issue has been completed by @Timothy2025-20 as part of the Stellar Wave Program's 5th Wave 🥳

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