fix: include hint and docs URL in FSTWRN004 warning message

[aquie00t]

Summary

The FSTWRN004 warning was created in lib/warnings.js with a static
message that contained no %s format placeholder. As a result, the
hint string passed at the call site in fastify.js was silently dropped
by process-warning, and users who triggered the warning never saw the
actionable information about allowErrorHandlerOverride.

Root Cause

process-warning's createWarning only interpolates arguments when the
message contains %s placeholders. With no placeholder present, any
argument passed to the emitter function is a no-op.

Changes

• lib/warnings.js: Embed the hint and docs URL directly into the
  static message of FSTWRN004.
• fastify.js: Remove the now-unused string argument from the
  FSTWRN004() call.
• test/set-error-handler.test.js: Add a test that asserts the
  warning is emitted with name, code, and the docs URL in the
  message when setErrorHandler is called more than once in the same
  scope.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[aquie00t]

@jean-michelet ready to merge whenever you are 🙏