Add test fixtures for `self-transfer` check (safe + vulnerable crates)

[lonerthefirst3-sudo]

Summary

The self-transfer check (crates/checks/src/transfer.rs) has inline unit tests but no standalone fixture crates under test-contracts/.

What to add

1. test-contracts/self-transfer-vulnerable/src/lib.rs — a transfer method that does not check from != to.
2. test-contracts/self-transfer-safe/src/lib.rs — the same method with an assert!(from != to) or equivalent guard.
3. Cargo.toml for each.

Acceptance criteria

• Vulnerable fixture produces at least one self-transfer finding
• Safe fixture produces zero findings
• cargo check passes inside each fixture directory

[temitope-007]

@temitope-007 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer

I'd like to take up this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @temitope-007 to this issue.

[starkiVector-090]

@starkiVector-090 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Can I work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @starkiVector-090 to this issue.

[drips-wave]

Congratulations, @starkiVector-090! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @starkiVector-090: 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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