[RELAYER] Harden RELAYER_USE_MOCK_SUBMISSION: forbid in production NODE_ENV and never skip validation

[wheval]

Feature Description

Audit all mock-submission branches. Ensure validation (signature, nonce, transfer policy) always runs; mock only replaces chain submit. Refuse to start with mock enabled when NODE_ENV=production.

Problem Statement

#937 reported mock mode bypassing validation. Current executeRelay validates first then mocks submit, but other code paths and deploy configs may still be unsafe. Need hard guarantees + tests.

Proposed Solution

1. Startup guard: mock flag + production → fatal error.
2. Single code path: validate → optional mock submit.
3. Integration tests for flag matrix.
4. Document env in relayer README and ops runbook.

Impact Area

• Infrastructure
• Security

Priority

• Critical

Implementation Complexity

• Complex

Related

#937

[Okorie2000-code]

Hi Maintainer,

I'd like to work on this issue. I have experience with backend development, testing, and security-focused systems. I can submit a clean PR within 48 hours.

[nurudeenheroic]

Hello! I'd appreciate the opportunity to work on this issue.