Auto-dispatch to _static_sample when StaticCache is detected
When generate() detects a StaticCache in model_kwargs after cache
preparation, automatically route SAMPLE and GREEDY_SEARCH modes to
_static_sample. The dispatch is skipped when custom_generate is
provided, allowing benchmarks to force the old _sample path.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>