Implement SDK response snapshot testing for API contract verification

[Kingsman-99]

Overview

SDK method return shapes are tested implicitly by unit tests but never asserted against a locked snapshot. Add snapshot tests so any accidental breaking change to a returned object's shape fails CI immediately.

Acceptance Criteria

• Snapshot tests using Vitest's toMatchSnapshot() for all read methods: getInvoice, getLeaderboard, getInvoiceHistory, getInvoiceExt, getStats
• Snapshots stored in tests/snapshots/ directory and committed to the repo
• Snapshots use deterministic fixture data (no timestamps or ledger sequences that change)
• bigint fields serialized as strings in snapshots (Jest/Vitest snapshot format)
• Snapshot update command: vitest run --update-snapshots (documented in CONTRIBUTING.md)
• CI fails if any snapshot diverges without an explicit update commit
• Snapshot diff shown in PR comment when snapshots change (via GitHub Actions)

[nonchalanttee22-lgtm]

@nonchalanttee22-lgtm has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello ill like to work on this issue.

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[Talentgift24-ux]

@Talentgift24-ux has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would love to work on this issue

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[attyolu]

@attyolu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Good morning Maintainer. I would love to resolve this issue for you in no time. Please assign to me. Thank you

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[ambermartin681]

@ambermartin681 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello dear maintainer i would love to work along side you on this task sir.

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[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, can I work on this issue?

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[grantfox-oss]

🦊 GrantFox — @ambermartin681 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #393)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @ambermartin681! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @ambermartin681: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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