fix: patch litellm ImageURLListItem to make index field optional

[nabinchha]

Summary

LiteLLM's ImageURLListItem TypedDict defines index as a required field, but some providers (e.g. OpenRouter) no longer include it in image responses. This causes a Pydantic validation error when LiteLLM constructs its Message object:

pydantic_core._pydantic_core.ValidationError: 1 validation error for Message
images.0.index
  Field required [type=missing, input_value={'type': 'image_url', 'im...}, input_type=dict]

Root cause

1. litellm/types/llms/openai.py — ImageURLListItem is a TypedDict with index: int as a required field (not total=False, no NotRequired)
2. litellm/types/utils.py — Message.images is typed as Optional[List[ImageURLListItem]]
3. litellm/litellm_core_utils/llm_response_utils/convert_dict_to_response.py — When constructing the Message, images from the provider response are passed through, but OpenRouter returns images like {"type": "image_url", "image_url": {"url": "data:image/png;base64,..."}} without an index field
4. Pydantic validates the ImageURLListItem TypedDict and fails because index is required but missing

Fix

Added patch_image_url_list_item() to litellm_overrides.py which monkey-patches index to NotRequired[int]. This is called during apply_litellm_patches().

Test plan

• Unit test test_patch_image_url_list_item_makes_index_optional verifies Message can be constructed without index
• E2E: run image generation with an OpenRouter image model