feat: add headers to all responses + ResendError

[drish]

Summary by cubic

Add HTTP response headers to all JSON dict responses under a headers key and surface headers on ResendError. Clients can read x-request-id and rate limit data; top-level array responses stay the same.

• New Features
  • Inject headers on all dict responses (including list wrappers).
  • Added BaseResponse; all response TypedDicts now extend it (headers optional).
  • Errors now include headers; ResendError and subclasses expose values like retry-after.

^{Written for commit f2e67d2. Summary will update on new commits.}