StylistService.cs: Unsafe array access on OpenAI response Content[0]

[AB-Law]

Problem

PluckIt.Infrastructure/StylistService.cs:68 directly accesses result.Content[0].Text without checking if Content is non-empty. If OpenAI returns an empty content array (on refusal or model error) this throws an unhandled IndexOutOfRangeException.

var content = result.Content[0].Text; // throws if Content is empty

Impact

Any stylist recommendation request that receives a non-standard OpenAI response crashes with a 500 and gives the user no actionable message.

Proposed Fix

if (result.Content is not { Count: > 0 })
    throw new InvalidOperationException("OpenAI returned empty content.");
var content = result.Content[0].Text;

Functionality Impact

Converts silent crash to explicit structured error response.