feat(isolation): DynamicVRAM compatibility for process isolation

DynamicVRAM's on-demand model loading/offloading conflicted with  process isolation in three ways: RPC tensor transport stalls from mid-call GPU offload, race conditions between model lifecycle and active RPC operations, and false positive memory leak detection from changed finalizer patterns.

- Marshal CUDA tensors to CPU before RPC transport for dynamic models
- Add operation state tracking + quiescence waits at workflow boundaries
- Distinguish proxy reference release from actual leaks in cleanup_models_gc
- Fix init order: DynamicVRAM must initialize before isolation proxies
- Add RPC timeouts to prevent indefinite hangs on model unavailability
- Prevent proxy-of-proxy chains from DynamicVRAM model reload cycles
- Add torch.device/torch.dtype serializers for new DynamicVRAM RPC paths
- Guard isolation overhead so non-isolated workflows are unaffected
- Migrate env var to PYISOLATE_CHILD

tests/execution/test_jobs.py

@@ -38,13 +38,13 @@ class TestIsPreviewable:
     """Unit tests for is_previewable()"""
 
     def test_previewable_media_types(self):
-        """Images, video, audio, 3d media types should be previewable."""
-        for media_type in ['images', 'video', 'audio', '3d']:
+        """Images, video, audio, 3d, text media types should be previewable."""
+        for media_type in ['images', 'video', 'audio', '3d', 'text']:
             assert is_previewable(media_type, {}) is True
 
     def test_non_previewable_media_types(self):
         """Other media types should not be previewable."""
-        for media_type in ['latents', 'text', 'metadata', 'files']:
+        for media_type in ['latents', 'metadata', 'files']:
             assert is_previewable(media_type, {}) is False
 
     def test_3d_extensions_previewable(self):