Fixes google#2111. As filed by knqyf263, 429 responses are never retried by
the default retry transport even though TooManyRequestsErrorCode is
already classified temporary in transport/error.go.

The retry transport in pkg/v1/remote/transport/retry.go short-circuits
on its codes list before consulting Temporary(); since 429 isn't in
defaultRetryStatusCodes, the conversion to a temporary transport.Error
never happens and retry.IsTemporary never gets called.

The fix is symmetric across the two parallel classifications:

  * defaultRetryStatusCodes in pkg/v1/remote/options.go now includes
    http.StatusTooManyRequests, so the retry transport wraps a 429
    response into a temporary *transport.Error and retries it through
    the existing backoff (Duration 1s, Factor 3, Steps 3).

  * temporaryStatusCodes in pkg/v1/remote/transport/error.go also
    gains http.StatusTooManyRequests, bringing the raw-status fallback
    path into alignment with temporaryErrorCodes which has already
    been classifying TooManyRequestsErrorCode as temporary.

The exhausted *transport.Error still carries StatusCode == 429, so
pkg/gcrane/copy.go's hasStatusCode helper continues to route 429s
through GCRBackoff at the application layer. The two retry layers
compose: ~1.3s of transport-level retry per request, then ~6s+ of
copy-level retry per image. No existing test expectations change.

Out of scope here: honoring the Retry-After response header. The retry
backoff is currently static; honoring server-dictated delays would
need plumbing through the retry/wait package. Worth a follow-up.

Tests:

  * pkg/v1/remote/transport/retry_test.go gains
    TestRetryTransport_TooManyRequests, which exercises three 429s
    through the retry transport and asserts both that all three
    attempts are made and that the final response surfaces with
    StatusCode == 429 (so gcrane's hasStatusCode keeps matching).
    It also pins the new entry in temporaryStatusCodes.

  * pkg/v1/remote/options_test.go (new file) pins 429 in the default
    retry status code list. A future refactor that drops 429 will
    fail loudly before the regression ships.

Verified locally:
  go test ./... (full suite green)
  gofmt + goimports clean
  golangci-lint v2.11: 0 issues
  woke clean on touched files
  reviewdog/action-misspell US: clean on the diff