fix(responses): default missing InputImageContent.detail during deserialization

[ajcasagrande]

• default InputImageContent.detail to auto when missing in JSON input
• add focused response-types integration tests covering both the
  EasyMessage and strict Item::Message(Input::User) deserialization paths

Closes #545.

InputImageContent.detail is documented as defaulting to auto, but without
#[serde(default)] any spec-compliant payload that omits detail (per
openai-python's ResponseInputImageParam, which marks detail
Optional[Literal["low","high","auto"]]) fails to deserialize. Because
InputItem is #[serde(untagged)], the single missing-field error rolls up
to data did not match any variant of untagged enum InputItem, which is
what spec-compliant clients hit in practice (full traceback in #545).

This mirrors the precedent set by #527 (EasyInputMessage.r#type,
WebSearchApproximateLocation.r#type) and uses the same test file
(tests/responses_input_item_serde.rs).

ImageDetail already derives Default with #[default] Auto, so the
fix is a single #[serde(default)] attribute — no Default impl needed.

Verification

• cargo test -p async-openai --test responses_input_item_serde --features response-types → 4 passed (2 pre-existing + 2 new)
• cargo fmt --check → clean
• cargo clippy -p async-openai --features response-types --tests -- -D warnings → clean
• End-to-end repro: against released v0.38.1 from crates.io, the payload below errors with data did not match any variant of untagged enum InputItem; against this branch the same payload deserializes successfully with detail defaulted to ImageDetail::Auto.

{
  "role": "user",
  "content": [
    {"type": "input_text", "text": "describe this"},
    {"type": "input_image", "image_url": "https://example.com/cat.png"}
  ]
}

Refs:

• openapi.documented.yml: ResponseInputImage (type required; image_url / file_id / detail all optional)
• openai-python: src/openai/types/responses/response_input_image_param.py — detail: Optional[Literal["low", "high", "auto"]]
• openai-node: src/resources/responses/responses.ts — detail?: 'low' | 'high' | 'auto'
• Prior fix in same spirit: fix(responses): default missing input type fields during deserialization #527

[64bit]

Thanks for details and tests, appreciate it all!

PS: I still see detail to be Required