common : auto-download dflash- and eagle3- HF sidecars (#25811)

common : auto-download dflash- and eagle3- HF sidecars (#25811)

* common: auto-download dflash- and eagle3- HF sidecars

Mirror the existing mtp- sidecar logic to support auto-discovery and
download of DFlash (dflash-) and Eagle3 (eagle3-) speculative decoding
sidecars from Hugging Face repos.

Changes:
- Add --dflash and --eagle3 CLI flags to trigger sidecar download
- Add find_best_dflash() and find_best_eagle3() using find_best_sibling
- Exclude dflash- and eagle3- filenames from primary model selection
- Filter dflash- and eagle3- from cached model listings
- Wire download tasks that set speculative.draft.mparams as fallback

Assisted-by: pi:llama.cpp/Qwen3.6-27B
* docs : regen