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.

[ogazboiz]

the payload-limit code is correct: express.json({ limit: "100kb" }) in app.ts, and errorHandler returns a structured 413 on entity.too.large, placed before the generic 500 so the body-parser error reaches it. the errorHandling suite passes 15/15 including the new over-limit test, and the app never uses urlencoded parsing so there's no gap there. two things:

1. the branch is stale (behind main, pre-esm-conversion), so the red ci is stale-base not your change. please rebase: git fetch origin && git rebase origin/main && git push --force-with-lease.
2. drop the stray pr_body_1184.md from the repo root.

rebase + drop the md and this is good.

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