fix(oci): convert image_url to camelCase for OCI vision requests

[fede-kamel]

Summary

Fixes #760 — OciClientV2.chat() with image content returns 400 on Command A Vision.

Root Cause

OCI Generative AI expects camelCase field names. The SDK already transforms roles ("user" → "USER"), content types ("text" → "TEXT"), and tool types ("function" → "FUNCTION" in #758), but passes image_url through as snake_case instead of converting to imageUrl.

SDK sent:

{"type": "IMAGE_URL", "image_url": {"url": "data:image/png;base64,..."}}

OCI expects:

{"type": "IMAGE_URL", "imageUrl": {"url": "data:image/png;base64,..."}}

Fix

Convert image_url → imageUrl in content items during request transformation.

Verified

response = client.chat(
    model="command-a-vision",
    messages=[{
        "role": "user",
        "content": [
            {"type": "text", "text": "What color is this image? Reply with one word."},
            {"type": "image_url", "image_url": {"url": "data:image/png;base64,..."}},
        ],
    }],
)
# Response: "Red"

Integration test added

test_chat_vision_v2 sends a 1x1 red PNG to Command A Vision and asserts:

• Response is not None
• Content is non-empty
• Model correctly identifies the color as "red"

tests/test_oci_client.py::TestOciClientV2::test_chat_vision_v2 PASSED
================ 46 passed, 13 deselected, 71 warnings in 7.36s ================

Test plan

• 46/46 non-streaming tests pass
• New test_chat_vision_v2 passes against OCI us-chicago-1 with command-a-vision
• Existing content transformation tests unaffected

@daniel-cohere Vision was broken on OCI because image_url wasn't converted to camelCase imageUrl. Same pattern as the tool type issue in #758.

---

Note

Low Risk
Low risk, localized request-shaping change for OciClientV2.chat() plus a new integration test; primary risk is limited to OCI vision payload compatibility/regression for message content transformation.

Overview
Fixes OciClientV2 chat request transformation to convert vision content items from snake_case image_url to OCI-required camelCase imageUrl, preventing 400s on Command A Vision.

Adds an integration test (test_chat_vision_v2) that sends a tiny inline PNG via image_url content and asserts the model returns a non-empty response that identifies the color.

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[fede-kamel]

@daniel-cohere Conflicts resolved and all 65 integration tests passing against live OCI GenAI (including the new vision test). Ready for merge whenever you are.