Add SDK deterministic integration fixture framework

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Create deterministic fixtures for SDK integration tests covering accounts, balances, payments, transactions, network responses, Soroban calls, and vault actions.

Current Behaviour

Tests may depend on live network state or scattered mock objects.

Expected Behaviour

The SDK should use reusable deterministic fixtures for success, failure, timeout, unsupported feature, and unknown-status cases.

Suggested Implementation

Create fixture conventions, helper builders, baseline fixtures, invalid examples, docs, and refactor existing tests to use them.

Files or Areas Likely Affected

• tests/
• src/accounts/
• src/payments/
• src/transactions/
• src/network/
• src/soroban/
• src/vault/
• docs/

Acceptance Criteria

• Fixture framework is implemented.
• Account, payment, transaction, network, and vault fixtures are included.
• Failure, timeout, and unsupported-feature cases are included.
• Tests avoid live-network dependency where practical.
• Documentation explains how contributors add fixtures.
• Existing relevant tests use the shared framework.

Additional Notes

This should improve test reliability.

[peter-j23]

hi maintainer,

am a software engineer with years of experience and an enthusiast in open source projects, i will like to resolve this issue if assigned to me. Thank you

[grantfox-oss]

🦊 GrantFox — @peter-j23 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #281)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@peter-j23's PR #451 was approved and merged by @El-swaggerito.

🏆 @peter-j23: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (589 total points). Track your full progress on GrantFox.

👏 Great work, @peter-j23! Keep contributing to Axionvera.