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/api/endpoints.py

@@ -15,9 +15,9 @@ def _format_tags(raw_tags):
 
 def get_samplers():
     from modules import sd_samplers_diffusers
-    all_samplers = []
+    all_samplers = [{'name': 'Default', 'options': {}}] # restores the model's own scheduler; first to match the ui dropdown order
     for k, v in sd_samplers_diffusers.config.items():
-        if k in ['All', 'Default', 'Res4Lyf']:
+        if k in ['All', 'Default', 'Res4Lyf']: # remaining keys are shared config templates, not samplers
             continue
         all_samplers.append({'name': k, 'options': v})
     return all_samplers