Add property-based fuzz tests for stream financial invariants

[ogazboiz]

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Description

All stream tests use handpicked values. Fuzzing with random inputs is the most effective way to catch edge cases in the streaming math before mainnet deployment.

Invariants to Fuzz

• withdrawn <= deposited — recipient never receives more than deposited
• claimable <= (deposited - withdrawn) — claimable never exceeds remaining
• cancel_refund + amount_withdrawn <= deposited — total payout never exceeds deposit
• Accrual after multiple pause/resume cycles equals sum of active-period durations × rate
• After protocol fee: net_deposited == gross × (1 - fee_rate_bps / 10_000)

Use soroban-sdk testutils with random amount (1..i128::MAX/2), duration (1..u32::MAX), varied pause points.

Files

• contracts/stream_contract/src/test.rs

Acceptance Criteria

• Fuzz tests cover all 5 invariants
• 10,000+ iterations with no violation
• cargo test clean

[overprodigy]

@overprodigy has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I'd liek to work on this.

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