feat: cap max invoices per business

[AugistineCreates]

This pr closes #1503

Summary

Implemented a defense‑in‑depth limit on the number of active invoices a business can create. Added a documented constant, exposed a getter, and enforced the cap in store_invoice. Updated tests with a negative case verifying the limit enforcement.

Threat Mitigation

Auditors could flag the absence of a per‑business invoice cap as a potential vector for denial‑of‑service or state‑exhaustion attacks. This change closes that gap by ensuring no business can exceed the configurable maximum, preventing abuse and protecting protocol resources.

Changes

• Added MAX_INVOICES_PER_BUSINESS constant and getter in protocol_limits.rs.
• Integrated limit check in contract.rs during invoice storage.
• Added comprehensive test test_store_invoice_respects_cap in test_max_invoices_per_business.rs.
• Updated documentation comments.

Verification

• All contract unit tests pass (cargo test -p quicklendx-contracts).
• Linting (cargo clippy -- -D warnings) and formatting (cargo fmt) succeed.

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