test: cover validate milestone scoped authorization

[dangelo352]

Summary

• add scoped-auth tests for validate_milestone with Some(verifier), None, and Some(creator)
• prove stranger-signed validation attempts fail and leave vaults active/unvalidated
• cover authorized success paths and the expired-vault auth-before-expiry ordering
• document the negative authorization coverage in TESTING_GUIDE.md and TEST_VERIFIER_SAME_AS_CREATOR.md

Closes #228

Validation

• cargo test --test validate_auth -- --nocapture
• cargo test
• cargo fmt --check
• git diff --check

[1nonlypiece]

thanks for jumping on this. #228 hasn't been assigned to you though, and we need the PR author claimed on the issue before review. closing as invalid for now — grab #228 on the board and raise it again. https://discord.gg/xvNAvMJf