[BACKEND] Add request IDs to structured validation and server error payloads consistently

[utksh1]

Summary

Request IDs are useful only if clients can correlate them with failures. SecuScan should make that correlation obvious in error payloads as well as headers.

Scope

• Decide where request IDs belong in structured error responses.
• Apply the contract consistently across validation errors and server errors.
• Add regression tests for the chosen shape.

Acceptance criteria

• Error payloads follow a documented request ID contract.
• Validation and server errors behave consistently.
• Tests cover the chosen response shape.

[Rakshak05]

Hii @utksh1, I would like to work on this. Can you please assign it to me under GSSoC'26 contributor!?

[anshul23102]

Interested in contributing to this under NSoC'26. This is a valuable issue addressing important functionality/quality. Happy to take this on.

/assign

[anshul23102]

Interested in contributing under NSoC'26. This is a valuable issue. Happy to work on it.

/assign

[Rakshak05]

Hii @utksh1, i've raised a PR for this issue. please check it out!