Add cancel_vault refund balance-assertion tests aligned with real token transfer

[1nonlypiece]

📌 Description

README_CANCEL_VAULT.md still describes cancel_vault as refunding a simulated
per-vault balance stored under ("vault_balance", vault_id), but the current
src/lib.rs performs a real Soroban token::Client::transfer from the
contract back to the creator. The tests should assert the real on-ledger refund,
and the doc should be corrected to match.

Goal: add tests asserting cancel_vault credits the creator by exactly
vault.amount and zeroes the contract's escrow, and update the cancel doc.

🎯 Requirements and Context

• Must be deterministic and isolated.
• Assert creator balance increases by vault.amount and contract balance returns
  to its pre-create value.
• Confirm the vault_cancelled event is emitted.
• Confirm cancel is rejected on non-Active vaults (Error::VaultNotActive).

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b feature/cancel-vault-refund-tests

2. Implement changes

• Tests: extend tests/lifecycle.rs with refund balance-delta assertions using
  TokenClient::balance.
• Docs: rewrite README_CANCEL_VAULT.md to describe the real token transfer
  (remove the "simulated balance" language) and add a docs/CANCEL.md if helpful.

3. Test and commit

• Run cargo test and regenerate coverage.
• Cover edge cases: cancel right after create, cancel on completed/failed/cancelled
  (reject), non-creator cancel (reject), double cancel (reject).

Example commit message

test+docs: real-transfer refund coverage for cancel_vault

✅ Guidelines

• Minimum 95% test coverage on cancel_vault lines.
• Clear, corrected documentation in README_CANCEL_VAULT.md.
• All tests deterministic and passing.
• Timeframe: 96 hours.

🏷️ Labels

type-testing · area-contracts · type-documentation · MAYBE REWARDED ·
GRANTFOX OSS · OFFICIAL CAMPAIGN

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