test(contract): add fuzz coverage for conversion, fee, and overflow invariants

[arisu6804]

Problem

Example-based tests do not cover the large input space where rounding, overflow, and reserve edge cases can break financial invariants.

Objective

Deliver a production-quality improvement to math and state-transition test suite that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Generate valid and invalid asset, share, fee, and rate combinations.
• Assert conservation, monotonicity, bounded fees, and safe rejection of overflow inputs.
• Keep seeds and minimized counterexamples reproducible in CI.

Acceptance criteria

• Fuzz runs cover all core conversion and fee paths.
• Any counterexample is reported with a reproducible seed and input.
• No invariant is weakened to make the suite pass.

Required validation

• Deterministic fuzz/property tests and regression fixtures for every discovered boundary.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused on this issue and explain design tradeoffs.
• Include test evidence, compatibility impact, and migration or rollout notes where relevant.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.