Feat: whale- guard implementation.

[YuvikaLimboo]

📋 Summary
This update adds a security check to the "Human-in-the-Loop" (HITL) system. It automatically flags large transactions for manual approval and adds a way to track the history of a transaction so we can verify it later.

🔗 Related Issues
Closes #1658

🧠 Context
When the system handles money or important data, we shouldn't let large amounts move without a person double-checking. This change creates a "safety trigger" for big transactions and ensures we keep a clear record (an audit trail) of who did what and when. This is standard for any professional financial or blockchain software.

🛠️ Changes
Large Transaction Check: Added a rule that stops any transaction over 100 SOL and asks for a manual review.

Better Error Messages: Added specific labels for errors, such as when a digital signature is wrong or when a record is missing important information.

Record Keeping: Added a requirement for an "audit trail" so the system knows the history of every request.

System Cleanup: Made sure these new errors talk to the main engine correctly so the whole program doesn't crash when a check fails.

🧪 How you Tested
I tested these changes using the built-in testing tools in the crates/mofa-kernel/src/hitl/policy.rs file.

Test Commands: I ran cargo test hitl::policy::policy_tests.

Big Transaction Test: I verified that a 150 SOL transaction is stopped correctly.

Missing Info Test: I verified that if the record-keeping info is missing, the system catches it and shows an error.

📸 Screenshots / Logs
Plaintext
running 2 tests
test hitl::policy::policy_tests::test_audit_validation_policy_triggers ... ok
test hitl::policy::policy_tests::test_whale_guard_triggers_on_large_amount ... ok

test result: ok. 2 passed; 0 failed; finished in 0.33s
⚠️ Breaking Changes
[x] No breaking changes (This won't break existing code).

🧹 Checklist
[x] Code is clean and follows standard Rust rules.

[x] Ran cargo fmt to fix spacing and layout.

[x] All tests passed on my computer.

[x] The commit messages explain the reasoning behind the changes.

🧩 Additional Notes for Reviewers
I have set the limit for "large transactions" to 100 SOL for now. In the future, we can make this number adjustable, but for now, it is a hard limit to keep things safe.