fix(campaign): use typed Error::Overflow in calculate_refund_amount

[Bug-Hunter-X]

Overview

This PR fixes calculate_refund_amount in the campaign contract so refund arithmetic failures use the contract's typed Error::Overflow panic path instead of debug_assert! and .expect(). This ensures zero-denominator and overflow checks remain active in release/WASM builds and that off-chain clients can match failures against stable error discriminants.

Related Issue

Closes #33

Changes

🛡️ Typed Error Handling in Refund Math

• [MODIFY] campaign/src/lib.rs
• Replaced debug_assert!(refund_denominator > 0) with a runtime panic_with_error(env, Error::Overflow) check.
• Replaced .expect("overflow in refund numerator") with checked_mul(...).unwrap_or_else(|| panic_with_error(env, Error::Overflow)).
• Plumbed &Env through calculate_refund_amount and its claim_refund call site.
• Preserved the PR Integer division precision loss zeroes out small donor refunds #21 anti-dust floor behavior and comment.

🧪 Tests

• [MODIFY] campaign/src/test/claim_refund_tests.rs

• Added unit tests asserting Error(Contract, #17) on zero denominator, negative denominator, and integer overflow.

• Added integration tests through claim_refund for overflow and zero-denominator paths.

• Added anti-dust floor regression test.

• [ADD] campaign/test_snapshots/test/claim_refund_tests/*.json

• Added Soroban test snapshots for the new negative-path and anti-dust tests.

Verification Results

cargo test (campaign) ✅ passed (148/148)

Acceptance Criteria	Status
debug_assert! removed; replaced with typed panic_with_error	✅
.expect() replaced with unwrap_or_else + panic_with_error	✅
calculate_refund_amount accepts &Env	✅
Negative-path tests for overflow and zero denominator	✅
PR #21 anti-dust floor comment preserved	✅
All existing refund and invariant tests pass	✅