Add test for rounded-zero protocol fee

[Killerjunior]

Description
Add coverage for the untested collect_fee branch where fee_rate_bps > 0 but fee rounds down to zero for tiny amounts, ensuring behavior is explicitly verified.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #794

Changes Made

• Added a new unit test in contracts/stream_contract/src/test.rs:
  • test_no_fee_transfer_or_event_when_fee_rounds_to_zero
• Test initializes protocol with non-zero fee rate (200 bps) and creates a stream with tiny amount (1) so fee calculation truncates to zero.
• Added assertions to verify:
  • Treasury token balance remains 0
  • Stream deposited_amount remains full amount (1)
  • No fee_collected event is emitted
• Updated collect_fee doc comment in contracts/stream_contract/src/lib.rs to explicitly describe the rounded-zero-fee behavior.

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

1. Run:
   cargo test --manifest-path contracts/stream_contract/Cargo.toml test_no_fee_transfer_or_event_when_fee_rounds_to_zero
2. Confirm the new test validates rounded-zero fee behavior (treasury unchanged, full deposit retained, no fee event).
3. (Optional) Run full contract test suite from same manifest path.

Breaking Changes

Breaking Changes:

• None.

Migration Guide:

• No migration required.

Screenshots/Demo

N/A

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes

• Local test execution is currently blocked in this environment due a Windows linker/toolchain issue (link.exe failure; missing/incorrect C++ build tools setup), not due contract/test logic.
• Branch pushed: test-794-fee-rounds-to-zero
• Suggested PR URL: `https://github.com/Killerjunior/flowfi/pull/new/test-794-fee-rounds-to-zero