fix(types): omit ImageUrl.detail when None

[yourlogarithm]

Problem

ImageUrl.detail (async-openai/src/types/shared/image_url.rs) is an Option<ImageDetail> with no skip_serializing_if, so when it is None the request body carries an explicit "detail": null:

{ "type": "image_url", "image_url": { "url": "data:image/png;base64,…", "detail": null } }

The official OpenAI API tolerates this, but stricter OpenAI-compatible gateways validate detail as a Literal['auto','low','high'] and reject the explicit null with an HTTP 400:

1 validation error for ChatCompletionContentPartImageParam
image_url.detail
  Input should be 'auto', 'low' or 'high' [type=literal_error, input_value=None, input_type=NoneType]

Since detail is an optional field, the correct wire representation when unset is to omit it entirely (the server then applies its own default, auto).

Fix

Add #[serde(skip_serializing_if = "Option::is_none")] to ImageUrl.detail so the field is omitted when None. No behavioural change against the OpenAI API itself; it just stops emitting an explicit null that some compatible backends reject.

#[serde(skip_serializing_if = "Option::is_none")]
pub detail: Option<ImageDetail>,

Deserialization is unaffected (the field already round-trips), and explicit values (Some(Auto/Low/High)) still serialize as before.

[64bit]

Thank you for detailed description and PR!