fix(fetch): preserve timeout cause on body errors - #1126

Summary

• Preserve request timeout and cancellation causes when response-body cleanup races with transport errors.
• Apply the same normalization to response formatting and output streaming errors.

Validation

• gofmt check
• go test -v -parallel 8 ./...
• go test -race ./internal/fetch -run ^TestRetryBudgetCoversResponseBody -count=100
• go build .
• staticcheck ./...
• go vet ./internal/fetch