test(stream): add withdraw with zero claimable test

[devUnixx]

Closes Vera3289#54

Changes

• Added test_withdraw_zero_claimable_returns_zero in contracts/stream/src/test.rs

What the test covers

1. Creates a stream and advances the ledger by 100 seconds
2. First withdraw succeeds and returns 1000 (100s × rate 10)
3. Second withdraw at the same timestamp returns 0 — no token transfer occurs
4. Asserts stream state (withdrawn, status) is unchanged after the no-op call

This confirms the contract handles the zero-claimable path gracefully without panicking or issuing a zero-value token transfer.