bug: openai base64 image content rejects default image detail

[fangkangmi]

• I have looked for existing issues (including closed) about this

Bug Report

The OpenAI chat-completions converter rejects base64 image content when
Image.detail is None, even though rig::completion::message::Image exposes
detail as Option<ImageDetail> and ImageDetail::default() is Auto.

URL-backed images already treat None as ImageDetail::Auto, but base64-backed
images still return this conversion error:

OpenAI image URI must have image detail

That makes a type-valid image fail only at request-conversion time.

Reproduction

use rig::completion::message::{DocumentSourceKind, Image, ImageMediaType, UserContent};
use rig::providers::openai;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let image = Image {
        data: DocumentSourceKind::Base64("iVBORw0KGgo=".to_string()),
        media_type: Some(ImageMediaType::PNG),
        detail: None,
        additional_params: None,
    };

    let content = UserContent::Image(image);

    // This is the OpenAI provider conversion path used when building a chat
    // completions request from Rig message content.
    let result = openai::completion::UserContent::try_from(content);

    assert!(result.is_ok());
    Ok(())
}

Today the conversion fails with:

OpenAI image URI must have image detail

The same detail: None value is accepted for URL-backed image content because
that path uses detail.unwrap_or_default().

Expected behavior

Base64 image content should behave the same as URL-backed image content:

detail.unwrap_or_default()

So detail: None should serialize as OpenAI image detail auto.

Additional context

This came up while building a downstream multimodal grading flow. Our message
builder constructed image attachments with detail: None, which is valid at the
Rig type level, but OpenAI request conversion failed at runtime.

The local workaround is to always set:

detail: Some(ImageDetail::Auto)

But since ImageDetail already defaults to Auto, the OpenAI converter can
avoid this footgun by defaulting None in the base64 image path too.

[linear-code]

RIG-1317