Add test: dispute during active round with pending contributions

[big14way]

Test dispute behavior when some members have already contributed.

Acceptance Criteria:

• Start group, have some members contribute
• Raise dispute mid-round
• Verify contributions are preserved
• Resolve dispute and verify round can complete

Files: contracts/sorosave/src/test.rs

[er1c-cartman]

Submitted focused regression coverage for this in PR #147.

It starts an active group, records partial round contributions, raises a dispute before the final member contributes, verifies the pending round state is preserved during the dispute, resolves the dispute, and then completes the round normally.

Validation:

• cargo test --package sorosave
• cargo fmt --check
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check