Add test coverage script and enforce coverage thresholds

[Code-Paragon]

Problem

The jest.config.js sets up collectCoverageFrom, but package.json lacks a command to generate the report, and no minimum coverage threshold is enforced.

Expected behavior

A test:coverage script is added to package.json, and jest.config.js is updated to enforce at least 80% global coverage for branches, functions, and lines.

Files to update

• package.json
• jest.config.js

Project relevance

Enforces the project's strict testing requirements for financial middleware, mathematically guaranteeing code reliability.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• coverageThreshold is added to the Jest config enforcing a minimum of 80%
• No TypeScript, lint, test, or build errors