test : added unit tests for lib/request-id module - #6

Summary of What Has Been Done

Created lib/request-id.test.ts with unit tests covering:

• Returns the x-request-id header value when the header is present
• Returns null when the header is absent
• Returns null when the header value is an empty string
• Calls headers() with no arguments (no parameter passed)

Mocked next/headers using vi.mock.

Changes Made

• New file: lib/request-id.test.ts

Impact it Made

Adds first unit test coverage for the request-id module, supporting request tracing and debugging use cases.

Closes #8421.

Note: please assign this PR to the tmdeveloper007 account.