Studio: classify local video pipelines, harden video preflight, and gate custom-root sd.cpp removal on ownership

- _local_model_task now tags a local diffusers pipeline that resolves to a video
  family (LTX / Wan / Hunyuan) as text-to-video, mirroring the cached-repo
  _cached_repo_task, so supported local video pipelines surface in the Video
  On-Device picker instead of being routed to the Images picker where the image
  loader rejects them. Gated on _local_is_diffusers so only a real loadable
  pipeline dir reaches the video check.
- Video validate_load_request now rejects a local pipeline pick whose directory has
  no model_index.json before the GPU handoff, mirroring the image loader, so a bad
  local pipeline can no longer evict the resident model and only then fail deep in
  from_pretrained.
- The custom/env-mode uninstall now removes a sibling stable-diffusion.cpp only when
  it carries the Studio owner marker. install_sd_cpp_prebuilt writes the canonical
  .unsloth-studio-owned marker on install; uninstall.sh and uninstall.ps1 keep any
  unowned checkout (a user's own git clone of stable-diffusion.cpp beside a custom
  Studio root is no longer deleted). A pre-marker Studio build is left behind rather
  than a user file removed.

Adds regression tests: local video pipeline tagged text-to-video (and a video-named
non-pipeline dir stays untagged so it can never trigger a doomed pipeline load), the
video local-pipeline preflight rejection, the install ownership marker, and the
uninstall keeping an unowned sibling while removing an owned one.

studio/install_sd_cpp_prebuilt.py

@@ -442,6 +442,13 @@ def install(
         _make_executable(sd_server)
     if sd_server is not None:
         print(f"installed sd-server -> {sd_server}", flush = True)
+    # Ownership marker (the same one setup.sh/_is_studio_root use, and setup.ps1 writes into
+    # the Node sibling dir) so the uninstaller can tell a Studio-installed sd.cpp from a user's
+    # own stable-diffusion.cpp checkout beside a custom Studio root, and delete only ours.
+    try:
+        (target / ".unsloth-studio-owned").touch()
+    except OSError:
+        pass
     return sd_cli