fix(pathfinder): Linux .so glob fallback prefers newest (#1732)

[cpcloud]

Resolves #1732.

Summary

• cuda_pathfinder/cuda/pathfinder/_dynamic_libs/search_platform.py:
  _find_so_in_rel_dirs (the site-packages wheel lookup) now uses
  sorted(..., reverse=True) on its versioned-.so glob, matching the
  newest-first policy already used by LinuxSearchPlatform.find_in_lib_dir
  (the conda / CUDA_PATH lookup) and by the descriptor-driven
  already-loaded / system-search order in load_dl_linux/load_dl_windows.
• Added an inline comment at _find_so_in_rel_dirs documenting why the
  fallback exists, that a single match is expected in practice, the
  newest-first tie-breaker, and a pointer back to this issue.
• Appended the same issue pointer to the existing comment at
  LinuxSearchPlatform.find_in_lib_dir.
• cuda_pathfinder/tests/test_search_steps.py: added six tests (three per
  call site) locking in the newest-first policy: zero / one / multiple
  versioned matches at both the site-packages and conda/CUDA_PATH call
  sites, using libcudart.so.12 / libcudart.so.13 as the fixtures.

Behavioral changes to ambiguity handling (raising when multiple versioned
files coexist) remain deferred to #1038, consistent with the triage
decision recorded on #1732.

Original review thread:
#1693 (comment)

Test plan

• pixi run pytest tests/ from cuda_pathfinder/ — 966 passed, 4
  skipped (unchanged count except +6 new tests).
• Ruff check + format pass on changed files.
• Mypy on the changed source file — no new errors (same pre-existing
  set).
• Sanity-checked that the new multi-match test fails on the
  pre-change sort by reverting the keyword locally.

[rwgk]

Thanks!

[github-actions]

Doc Preview CI
Preview removed because the pull request was closed or merged.