Add a property test proving update_user_volume tier transitions are monotonic and never regress within a period

[Baskarayelu]

📋 Description

update_user_volume in fees.rs accumulates a user's transaction volume into UserVolumeData and drives their VolumeTier, which feeds get_tier_discount. The tier must be monotonic in accumulated volume — adding volume can only keep or raise the tier, never lower it within a period. This is not yet asserted as a property.

Why this matters: a non-monotonic tier would mean a user's fee discount could randomly drop after they transact more — a correctness and fairness bug. A property test over arbitrary volume sequences proves monotonicity holds.

🎯 Requirements & Context

• Add quicklendx-contracts/src/test_volume_tier_monotonic.rs under fuzz-tests.
• Property: applying a sequence of non-negative volume increments never lowers the resulting VolumeTier.
• Property: get_tier_discount is non-decreasing as tier rises.
• Property: tier boundaries are correct (just below vs just above each threshold).

Context & constraints

• proptest 1.4; bound increments to avoid overflow-checks panics on accumulation.

🛠️ Suggested Execution

git checkout -b test/volume-tier-monotonic
cargo test -p quicklendx-contracts --features fuzz-tests test_volume_tier_monotonic -- --nocapture
cargo clippy -p quicklendx-contracts --all-targets -- -D warnings

• Edge cases: increments of zero (no tier change); crossing exactly one threshold; saturating to the top tier.

Example commit message

test(fees): prove update_user_volume tier transitions are monotonic

Property test over arbitrary increment sequences asserting tier and discount
never regress.

✅ Acceptance Criteria & Guidelines

Requirement	Target
Tier + discount monotonicity asserted	Required
Boundary correctness covered	Required
Coverage of tier logic	≥ 95%
cargo clippy clean	Required
Timeframe	96 hours from assignment

💬 Community & Support

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