Fix OpenAI Codex vision support by converting file paths to data URIs
before sending requests to the API.
## Problem
OpenAI Codex API was rejecting vision requests with 400 error:
"Invalid 'input[0].content[1].image_url'. Expected a valid URL,
but got a value with an invalid format."
Root cause: provider was sending raw file paths (e.g. `/tmp/test.png`)
instead of data URIs (e.g. `data:image/png;base64,...`).
## Solution
Add image normalization in both `chat_with_system` and `chat_with_history`:
- Call `multimodal::prepare_messages_for_provider()` before building request
- Converts file paths to base64 data URIs
- Validates image size and MIME type
- Works with both local files and remote URLs
## Changes
- `src/providers/openai_codex.rs`:
- Normalize images in `chat_with_system()`
- Normalize images in `chat_with_history()`
- Simplify `ResponsesInputContent.image_url` from nested object to String
- Fix unit test assertion for flat image_url structure
- `tests/openai_codex_vision_e2e.rs`:
- Add E2E test for second profile vision support
- Validates capabilities, request success, and response content
## Verification
✅ Unit tests pass: `cargo test --lib openai_codex`
✅ E2E test passes: `cargo test openai_codex_second_vision -- --ignored`
✅ Second profile accepts vision requests (200 OK)
✅ Returns correct image descriptions
## Impact
- Enables vision support for all OpenAI Codex profiles
- Second profile works without rate limits
- Fallback chain: default → second → gemini
- No breaking changes to existing non-vision flows
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
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