v0.40.8: mlx: add Flint model support (#19181)

Summary

Preserve source tool JSON alongside decoded API values so Flint can retain numeric types and schema order in prompts. Existing API marshaling is unchanged. Assemble ModelOpt expert banks during safetensors import, and reject direct Flint NVFP4 creation before conversion. Other models retain their existing import and quantization paths. Add optional learned sinks and device causal masks to shared MLX attention. Existing callers retain the no-sink and host-mask paths; Flint opts into the new behavior.

Lotu Radar provides attributed news summaries and links to the original publisher. Full reporting and copyright remain with the source.