fix: reject bounded streams whose total obligation overflows

[Goodness-0x]

Fix: reject bounded streams whose total obligation overflows

Summary

Fixes #405 by rejecting bounded DripStream initializations whose total obligation would overflow i128.

Previously, initialize() validated zero/negative rates and malformed time ranges, but it did not reject the case where:

rate_per_second * (end_time - start_time)

would overflow. That allowed a direct-initialized stream to be created in a state that later trapped settlement flows (withdraw, cancel, clawback, force_cancel) with ArithmeticOverflow after enough ledger time elapsed, permanently locking the escrow.

What changed

• Added an upfront bounded-stream overflow check in lib.rs
• Added a focused regression test in tests.rs covering the overflow case

Verification

I verified with:

source "$HOME/.cargo/env" && cd /workspaces/streamFi-contracts && cargo test -p drip-stream -- --nocapture

Closing

Closes #405

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