fix(errors): defensive map copies in WithFields/WithDetails; remove C…

[KARTIKrocks]

…odeTooManyRequests

Summary

Changed

• errors — WithFields and WithDetails now copy the caller-supplied map before storing it, preventing external mutation of the error's internal state after the call

Removed

• errors — CodeTooManyRequests constant and its codeStatusMap entry removed; rate-limit errors are owned by the middleware package

Fixed

• errors — Added clarifying doc comment on (*Error).Is explaining that matching is code-based and symmetric between two *Error values with the same Code

Checklist

• fmt, vet, lint, test, build passes (make all)
• New code has tests where appropriate
• Breaking changes are documented