test+docs: cover cancel vault refunds

[dangelo352]

Closes #232

Summary

• add lifecycle coverage that asserts cancel_vault refunds exactly vault.amount to the creator and returns contract escrow to its pre-create balance
• assert success/failure destinations stay untouched on cancellation
• add terminal-state cancellation checks for completed, failed, and already cancelled vaults returning Error::VaultNotActive
• add an internal event test for the vault_cancelled event
• update README_CANCEL_VAULT.md and TESTING_GUIDE.md to describe real Soroban token-transfer refund behavior instead of simulated balance storage

Validation

• cargo test test_cancel_vault — passed
• cargo test — passed, 69 tests total across unit/integration/property suites
• git diff --check

Notes

• cargo fmt completed; stable rustfmt prints warnings that this repo's imports_granularity and group_imports settings are nightly-only.

[1nonlypiece]

thanks for jumping on this. #232 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 #232 on the board and raise it again. https://discord.gg/xvNAvMJf