feat(preview): add per-queue live preview method override (#11261)

- Add set_preview_method() to override live preview method per queue item
- Read extra_data.preview_method from /prompt request
- Support values: taesd, latent2rgb, none, auto, default
- "default" or unset uses server's CLI --preview-method setting
- Add 44 tests (37 unit + 7 E2E)

latent_preview.py

@@ -8,6 +8,8 @@ import folder_paths
 import comfy.utils
 import logging
 
+default_preview_method = args.preview_method
+
 MAX_PREVIEW_RESOLUTION = args.preview_size
 VIDEO_TAES = ["taehv", "lighttaew2_2", "lighttaew2_1", "lighttaehy1_5"]
 
@@ -125,3 +127,11 @@ def prepare_callback(model, steps, x0_output_dict=None):
         pbar.update_absolute(step + 1, total_steps, preview_bytes)
     return callback
 
+def set_preview_method(override: str = None):
+    if override and override != "default":
+        method = LatentPreviewMethod.from_string(override)
+        if method is not None:
+            args.preview_method = method
+            return
+    args.preview_method = default_preview_method
+