fix(sampler): record and restore Default instead of the None sentinel

The keep-current-scheduler sentinel leaked into user-facing surfaces:
infotexts and filenames recorded Sampler: None, restoring such an
infotext pushed an invalid value into the dropdown, and the API
samplers list never offered Default at all.

- infotext always records the sampler, mapping a stray None to Default
- parse maps legacy Sampler: None infotexts to Default
- api samplers list leads with Default; the remaining excluded config
  keys are shared templates, not samplers

modules/processing_info.py

@@ -54,7 +54,7 @@ def create_infotext(p: StableDiffusionProcessing, all_prompts=None, all_seeds=No
         # basic
         "Steps": p.steps,
         "Size": f"{p.width}x{p.height}" if hasattr(p, 'width') and hasattr(p, 'height') else None,
-        "Sampler": p.sampler_name if p.sampler_name != 'Default' else None,
+        "Sampler": p.sampler_name if p.sampler_name != 'None' else 'Default',
         "Scheduler": shared.sd_model.scheduler.__class__.__name__ if getattr(shared.sd_model, 'scheduler', None) is not None else None,
         "Seed": all_seeds[index],
         "Seed resize from": None if p.seed_resize_from_w <= 0 or p.seed_resize_from_h <= 0 else f"{p.seed_resize_from_w}x{p.seed_resize_from_h}",