I suppose there might be an attorney out there who would be offended by a client asking for that; I wouldn't be nor would any attorney I know be offended by that either. I've had prospective clients ask about that and I don't get offended. Confidentiality is, after all, a legitimate concern for prospective clients and I don't expect them to know the rules of professional conduct. So, instead of chasing them off, I explain that the rules of professional conduct already mandate that I keep client AND prospective client information confidential.
You've had experience with an attorney who got ticked off by a client asking for a confidentiality agreement? As a client that would be a huge red flag of an attorney I'd not want to hire.