chore: revert rateLimiting API to avoid blocking release channel

[gabrielmfern]

It reverts most changes that were made in #562, but it doesn't revert the response mocking utilities that it introduced. This might interfere with the implementation in #611.

If we do decide to add this in later on, we can bring it back on another branch and also experiment further with the interface.

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Summary by cubic

Reverted the rateLimiting response API to unblock the release. All client methods now return only data and error, with no rate limit metadata.

• Refactors

  • Removed rate-limiting.ts and header parsing; dropped Response, RateLimit, and retryAfter types.
  • Updated all interfaces to a simple union: { data; error: null } | { data: null; error }.
  • Simplified Resend.fetchRequest error handling; no rate limit extraction.
  • Updated tests to stop asserting rateLimiting, replaced mockErrorResponse with fetchMock.mockOnce, kept mockSuccessResponse, and removed rate limit headers from mocks.

• Migration

  • Remove any usage of result.rateLimiting.
  • Stop relying on retryAfter on rate_limit_exceeded errors.
  • If tests used rate limit helpers or headers, update them to plain fetchMock.mockOnce with status and content-type.

[cubic-dev-ai]

No issues found across 42 files