fix: enforce 100kb payload limit on express.json and handle 413s

[SharifIbrahimDev]

Closes #1184

What does this PR do?

This PR enforces an explicit 100kb payload size limit on express.json() and correctly handles resulting entity.too.large errors so they return a structured 413 response rather than defaulting to an unhandled 500 error.

Description

• Explicit Size Limit: Added an explicit { limit: '100kb' } configuration to express.json() in app.ts. This protects the application and audit logs from unbounded payload sizes, while remaining more than generous enough to accommodate legitimate signed transaction payloads.
• Centralized Error Handling: Updated errorHandler.ts to natively catch entity.too.large errors emitted by body-parser and translate them into standard 413 Payload Too Large responses with the VALIDATION_ERROR code.
• Test Coverage: Added an integration test in errorHandling.test.ts to assert that a 150kb payload correctly trips the limit and returns the structured 413 error.
• Documentation: Added inline comments describing the rationale behind the payload limit in app.ts.