test: cover attestation fee accounting

[dangelo352]

Summary

• Add negative record_fees rejection coverage
• Add withdrawal tests for missing recipient, non-positive amounts, over-withdraw, and per-asset isolation
• Verify collected fee conservation against token balances after partial withdrawal
• Cover large bounded fee accumulation and withdrawal
• Document the attestation fee-accounting invariant in docs/FEE_MODEL_CROSS_CHECK.md

Closes #478.

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-attestation-fee-target cargo test -p attestation_engine fee -- --nocapture
• git diff --check