test: cover validated redirect guard

[dangelo352]

Closes #227

Summary

• add lifecycle coverage for create → validate → pass deadline → rejected redirect
• assert redirect_funds returns Error::NotAuthorized for a validated vault after the deadline
• prove failure_destination remains unchanged and release_funds still pays success_destination
• document the redirect guard invariant in TESTING_GUIDE.md
• include the stable rustfmt cleanup for tests/proptest_timestamps.rs that CI expects

Validation

• CARGO_TARGET_DIR=/tmp/disciplr-redirect-target cargo test test_validated_vault_cannot_redirect_after_deadline_and_can_release
• CARGO_TARGET_DIR=/tmp/disciplr-redirect-target cargo test — all tests passed
• git diff --check

[1nonlypiece]

thanks for the work here — but our flow is that you need to be assigned to the issue before opening a PR for it. #227 isn't assigned to you, so i'm closing this as invalid for now. please claim #227 first (comment on the issue or grab it on the GrantFox board), then re-raise and we'll review right away. https://discord.gg/xvNAvMJf