mtmd : DeepSeek-OCR image processing fixes, img_tool::resize padding refactor (#23345)

mtmd : DeepSeek-OCR image processing fixes, img_tool::resize padding refactor (#23345)

* mtmd : deepseek-ocr fixes, improvements and refactoring

- image processing changes to achieve full parity with Pillow (reference impl)
- SAM mask casting only when flash-attn is on
- SAM refactor (build_sam() extracted so deepseek-ocr-2 can reuse it)
- llama-chat changes to fix server/WebUI issue (new media_markers_first())
- adapted test-chat-template and added test cases for deepseek-ocr
- changed regression test for deepseek-ocr to use CER+chrF scores for ground-truth comparison; removed embedding-model
- ty.toml ignore unresolved-import for tools/mtmd/tests/**

* image-text reordering fix removed

* refactor bool add_padding + pad_rounding enum into a single pad_style enum