Add regression test that collect_fees after a prior collection with zero new accrual returns NoFeesToCollect

[Jagadeeshftw]

📌 Description

collect_fees resets FeesAccrued to 0 and returns Error::NoFeesToCollect if the balance is already 0. There is no explicit regression test confirming the second consecutive collect_fees call for the same asset (immediately after a first successful collection, with no new settlements executed in between) correctly fails with Error::NoFeesToCollect rather than, say, returning 0 successfully or double-counting.

🧩 Requirements and context

• Add a regression test that executes a settlement to accrue fees, calls collect_fees once successfully, then calls collect_fees again immediately for the same asset and asserts Error::NoFeesToCollect.
• Add a follow-up step: execute another settlement for the same asset after the first collection, and confirm a third collect_fees call now succeeds and collects only the newly-accrued amount, not anything from before.
• Confirm fees_collected events fire exactly once per successful collection, matching the accrued amount at that time.

🛠️ Suggested execution

• Add the test to src/test.rs near existing collect_fees coverage.
• Assert both the Result and the fees_accrued(asset) value after each step.
• No production code change expected — this closes a test-coverage gap on already-implemented behavior.

✅ Acceptance criteria

• A regression test confirms a second consecutive collect_fees call with no new accrual fails with Error::NoFeesToCollect.
• A regression test confirms a subsequent collection after new accrual collects only the new amount.
• fees_collected events accurately reflect each successful collection's amount.

🔒 Security notes

Fee-collection double-counting or an incorrect success-on-empty result would misstate protocol revenue in an off-chain accounting reconciliation, a low-severity but real correctness concern for anything treating collect_fees's return value as ground truth.

📋 Guidelines

• Minimum 95% test coverage
• Clear documentation
• Timeframe: 96 hours

[OluRemiFour]

Hi Maintainer,
Interested in working on this issue. I'll implement the requested changes and ensure everything is properly tested before submission.