feat: return response headers on all responses

[gabrielmfern]

Most of this is updating tests, the actual main changes are to resend.ts and interfaces.ts

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Summary by cubic

Return HTTP response headers on all SDK responses to improve debugging and integration visibility. Adds a headers field to response objects and unifies types via a new Response generic.

• New Features

  • Every response now includes headers: Record<string, string> | null (e.g., authorization, content-type).

• Refactors

  • Introduced Response type and replaced per-endpoint unions with this shared type.
  • Updated fetchRequest to attach headers on success and error responses.
  • Revised all interfaces and tests to use the new response shape.

[cubic-dev-ai]

No issues found across 80 files

[pkg-pr-new]

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npm i https://pkg.pr.new/resend/resend-node/resend@713

commit: 29cc01d