Add tokens.rs decimals self-test verifying the SAC reported decimals on registry

[greatest0fallt1me]

Description

When admins register a new SAC token, the on-chain registry trusts the declared decimals without verifying them. Add a self-test that calls the SAC's decimals() and rejects registration if the declared value does not match, preventing denomination mistakes that have caused real on-chain losses on other Stellar protocols.

Requirements and Context

• Cross-contract call inside tokens.rs
• Mismatch returns Error::TokenDecimalsMismatch
• Self-test runs on every register and on a manual re_verify_token admin entrypoint
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/token-decimals-self-test

2. Implement changes
   • contracts/predictify-hybrid/src/tokens.rs
   • contracts/predictify-hybrid/src/custom_token_tests.rs
3. Test and commit
   • cargo test -p predictify-hybrid token_decimals_self_test
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

feat: verify SAC decimals on token registration

Acceptance Criteria

• Mismatch rejected with specific error
• re_verify_token is admin-gated
• Test uses a mock SAC with mis-declared decimals

Guidelines

• Validate auth, overflow, and storage TTL/archival
• Clear documentation and inline comments
• Timeframe: 96 hours

[samuelfrancis163-eng]

@samuelfrancis163-eng 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 BECAUSE I AM CONFIDENT THAT I CAN DELIVER EFFICIENTLY
PLEASE ASSIGN ISSUE TO ME

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[aliyuHabibu]

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

Hi, please assign this issue to me. Thank you

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

[grantfox-oss]

🦊 GrantFox — @aliyuHabibu 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 #678)
2. Your PR will be reviewed by the Predictify-org maintainers

Good luck! Track your progress on GrantFox.

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

🧑‍💻 @aliyuHabibu: 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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