test: assert exact contract panic messages

[bobthecomputer]

Closes #49

Summary

• add a shared helper that extracts and exactly compares contract panic text from Soroban host errors
• cover every reachable business panic message across initialization, vesting creation, queries, claims, and revocation
• verify missing schedules through every public entry point that can return that error
• keep production contract behavior unchanged

Testing

• cargo test --locked: 15 passed, 0 failed
• rustfmt --edition 2021 --check contracts/vesting/src/test/mod.rs: passed
• cargo clippy --locked: passed with one pre-existing unused-import warning in contracts/vesting/src/lib.rs
• git diff --check: passed

The repository-wide cargo fmt --all -- --check currently reports pre-existing formatting drift in contracts/vesting/src/lib.rs; this PR keeps the change scoped to the test module.