test: enhance settlement and auth JWT integration tests

[Marvy247]

Summary

Enhances settlement.integration.test.ts and auth-jwt-validation.test.ts with comprehensive edge case coverage, error handling verification, and logging assertions.

Settlement integration tests (Issues #350, #351, #352)

• Edge cases: Non-existent invoice (404), zero/negative proceeds, wrong invoice status (settled, cancelled, draft), funded invoice with no confirmed investments
• ServiceError verification: Validates error codes (INVALID_PROCEEDS, INVOICE_NOT_FOUND, INVALID_INVOICE_STATUS, NO_CONFIRMED_INVESTMENTS) and HTTP status codes on failure paths
• Pro-rata distribution: Three-way uneven split, proceeds exceeding funded amount, proceeds less than funded amount, result structure validation
• Logging: Verifies lifecycle transition and settlement completion logs on success; verifies no logs emitted on failure paths
• Helper extraction: Added confirmInvestment and fullyFundInvoice utilities to reduce boilerplate

Auth JWT validation tests (Issue #353)

• Malformed tokens: Random non-JWT strings, two-part tokens (missing signature), empty strings, invalid base64 encoding
• Algorithm attacks: none algorithm, wrong HS256 secrets, alg-switch header tampering
• Missing claims: No sub claim, empty sub, non-existent user, non-string sub values
• Header edge cases: Lowercase bearer, raw token without prefix, Token scheme instead of Bearer, extra whitespace
• Error structure: Consistent error envelope verification across all failure scenarios, correct 401 status code (not 403/500)
• Timing: Token with future nbf, long-lived valid tokens

Acceptance Criteria

• Code in settlement.integration.test.ts is optimized and hardened
• Code in auth-jwt-validation.test.ts is optimized and hardened
• Backward compatibility maintained (existing tests preserved)
• No regression in existing functionality
• PR targets the dev branch

References

• Closes [Testing & QA] Enhance and optimize settlement.integration.test.ts (Issue #87) #350
• Closes [Testing & QA] Enhance and optimize settlement.integration.test.ts (Issue #88) #351
• Closes [Testing & QA] Enhance and optimize settlement.integration.test.ts (Issue #89) #352
• Closes [Testing & QA] Enhance and optimize auth-jwt-validation.test.ts (Issue #90) #353

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