Add Soroban test-snapshot hygiene check to contracts CI

[codebestia]

Add test_snapshots/ to contracts/.gitignore. Add a CI step after cargo test that fails if any test_snapshots/*.json files exist, since their presence indicates a test is producing host errors. If a test legitimately produces a snapshot, it should be explicitly allowlisted.

Acceptance criteria:

• contracts/.gitignore includes test_snapshots/
• CI step fails if any snapshot file exists after the test run
• Existing snapshot files removed from the repo in the PR adding this check
• A passing test suite produces no snapshot files

[DeveloperEmmy]

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

I’d like to work on this because it strengthens CI reliability and repository hygiene. I have experience improving Rust CI pipelines, automating quality checks, and maintaining clean, reproducible test workflows.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @DeveloperEmmy to this issue.

[drips-wave]

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

🧑‍💻 @DeveloperEmmy: 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.

Warning

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