Add automated code coverage threshold enforcement via Jest

[KingFRANKHOOD]

Summary

Configured global code coverage thresholds in Jest and added coverage report generation to CI pipeline. Also resolved pre-existing type errors and test failures.
Closes #128
Closes #129
Closes #130
Closes #131

Changes

DevOps

• jest.config.js: Added \coverageThreshold\ enforcing 60% branches, 80% lines/functions/statements
• api-cy.yml: Updated test step to run with --coverage\ flag; added coverage artifact upload step

Bug fixes (pre-existing errors resolved)

• stellar-address.utils.ts: Fixed \StrKey.isValidContract\ TypeScript type issue with type assertion
• invoice-state-machine.test.ts: Removed invalid second argument from \expect()\ call
• validate-invoice-for-publish.test.ts: Corrected expected error code from \FACE_VALUE_NOT_POSITIVE\ to \FACE_VALUE_TOO_LOW\
• stellar-address.test.ts: Replaced invalid test addresses with real valid Stellar keys

Verification

• Lint: ✅ passes
• Type check: ✅ passes
• Tests: 63 suites, 444 tests all passing
• Coverage: all thresholds met

Closes #

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