test(vault): claim when the balance is smaller than the tip

[enny791]

Closes #10

Summary

Adds two tests to contracts/rent_vault/src/test.rs covering the claim path when a vault's balance is at or below its tip — the state every underfunded vault ends in.

claim already guards this with VaultError::InsufficientBalance (207) when vault.balance < vault.tip, checked after the keeper/timing verifications and before any mutation or transfer. This PR asserts that behavior explicitly rather than leaving it implied.

What changed

• test::claim_rejects_underfunded_vault — funds a vault with 50 against a 100 tip, performs real maintenance (extend_all after a full interval), then claims. Asserts:
  • the outcome is InsufficientBalance (207, a 2xx error),
  • the vault balance is unchanged (50),
  • last_claim is unchanged (the failed claim does not consume the interval),
  • the keeper received 0 tokens and the vault still holds 50 — it never pays out more than it holds.
• test::claim_at_exact_tip_balance — funds exactly the tip (100), the boundary case of the balance check, then claims. Asserts a full payout of 100, a drained balance of 0, the token leaves the vault, and last_claim advances to the payout ledger.

Key design decisions

• No production code changed. The balance < tip check already runs before balance -= tip, so no underflow path exists (overflow-checks is on); the tests lock in that ordering.
• Used the existing rejection-test pattern (try_claim(...).err() == Some(Ok(VaultError::InsufficientBalance))) and the same Fixture/ledger-advance conventions as the rest of the module.
• The underfunded test records last_claim before the attempt and asserts it is unchanged, which is the direct evidence that a failed claim cannot silently consume the interval.

Acceptance criteria checklist

• A test funds a vault with less than tip, performs real maintenance, and claims — claim_rejects_underfunded_vault.
• The outcome is asserted explicitly as a specific 2xx error — InsufficientBalance (207).
• The vault balance after the attempt is asserted — 50, unchanged.
• last_claim is asserted — unchanged after the failed claim.
• A test covers a balance of exactly tip — claim_at_exact_tip_balance.

Test output

cargo test --all (rent_vault suite):

running 10 tests
test test::claim_at_exact_tip_balance ... ok
test test::claim_rejects_underfunded_vault ... ok
test test::balance_accounting_across_fund_claim_withdraw ... ok
... (all 10 pass)

test result: ok. 10 passed; 0 failed; 0 ignored

cargo fmt --all -- --check — clean. cargo clippy --all-targets -- -D warnings — clean. stellar contract build — complete (all three contracts build to wasm).

Follow-ups

None in scope. A partial-payout alternative was considered and rejected: paying a pro-rata amount would violate the fixed tip terms without a corresponding production change, so the existing explicit InsufficientBalance rejection is the behavior worth locking in.

Security note

No new trust assumptions or money-movement code paths are introduced; the changes are test-only. The tests confirm a vault can never pay out more than it holds and that a rejected claim does not advance the payout clock.