install: make the torch-index override authoritative across ROCm paths

Address review feedback on the override added in this PR so a pinned index is
honoured everywhere, not just in get_torch_index_url:

- Skip the WSL ROCm bootstrap (root privilege + large downloads, probes
  /dev/dxg) when UNSLOTH_TORCH_INDEX_URL / _FAMILY is set; it previously ran
  before the override was consulted.
- Skip the Radeon/Strix rerouting (which re-probes the GPU and overwrites the
  resolved URL with repo.radeon.com / repo.amd.com) when the index is pinned, so
  an explicit ROCm override (e.g. UNSLOTH_TORCH_INDEX_FAMILY=rocm6.4) is kept.
- install_python_stack.py: derive _TORCH_BACKEND from the override when
  UNSLOTH_TORCH_BACKEND is unset (standalone studio update), so _ensure_rocm_torch
  / _ensure_cuda_torch repair to the requested family instead of re-detecting.
- Strip ALL leading/trailing slashes in the shell override to match the Python
  side (avoids 404s on strict pip proxies).

Adds test cases for double-slash and leading/trailing-slash overrides.

tests/sh/test_get_torch_index_url.sh

@@ -415,6 +415,14 @@ assert_eq "url override beats family override -> url" "https://mirror.example.co
 _result=$(UNSLOTH_TORCH_INDEX_FAMILY="" UNSLOTH_TORCH_INDEX_URL="" run_func "none")
 assert_eq "empty overrides ignored -> detected cpu" "https://download.pytorch.org/whl/cpu" "$_result"
 
+# 46) ALL trailing slashes are stripped from a URL override (not just one).
+_result=$(UNSLOTH_TORCH_INDEX_URL="https://mirror.example.com/whl/cu128///" run_func "none")
+assert_eq "url override double slash stripped" "https://mirror.example.com/whl/cu128" "$_result"
+
+# 47) Leading and trailing slashes stripped from a family override.
+_result=$(UNSLOTH_TORCH_INDEX_FAMILY="//cu128//" run_func "none")
+assert_eq "family override slashes stripped" "https://download.pytorch.org/whl/cu128" "$_result"
+
 rm -f "$_FUNC_FILE"
 rm -rf "$_FAKE_SMI_DIR"
 rm -rf "$_TOOLS_DIR"