* Fix Codex PDF extraction fallback missing instructions
- add a Codex-specific systemPrompt on the PDF extraction fallback path
- keep non-Codex PDF fallback requests unchanged
- add regression coverage proving openai-codex-responses requests include instructions for PDF tool calls
* test: cover Codex text-only extraction fallback
- add regression coverage for the branch where PDF extraction includes images
but the selected Codex model only accepts text input
- assert Codex-specific extraction instructions are still attached in that path
* test: fix extracted image mock shape
- add the required `type: "image"` field to the text-only fallback regression mock
- keep the new Codex coverage test aligned with PdfExtractedImage
* test: align Codex PDF fallback tests
* docs(changelog): note PDF Codex fallback fix
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Co-authored-by: Dr JCai <jingxiao.cai@gmail.com>
Co-authored-by: anyech <8743351+anyech@users.noreply.github.com>
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