fix(pathfinder): remove dead/misleading error handling in platform loaders

[aryanputta]

Summary

Removes two dead error-handling paths in the dynamic-lib platform loaders. Both are unreachable / no-ops today; the Linux one additionally carries a factually wrong message that would mislead anyone debugging a load failure. This aligns with the cuda_pathfinder policy of not masking why discovery/loading failed.

Details

load_dl_linux.load_with_system_search guarded the resolved path with:

abs_path = abs_path_for_dynamic_library(desc.name, handle)
if abs_path is None:
    raise RuntimeError(f"No expected symbol for libname={desc.name!r}")

abs_path_for_dynamic_library never returns None - it returns a resolved path (os.path.join(l_origin, basename)) or raises OSError from dlinfo. So the branch is unreachable, and its message is wrong: it talks about a missing symbol when the only real failure mode is dlinfo path resolution. Dropped the dead branch so the descriptive OSError surfaces directly, and updated the docstring Raises: accordingly.

load_dl_windows.add_dll_directory had:

result = kernel32.AddDllDirectory(dirpath)
if not result:
    # Fallback: just update PATH if AddDllDirectory fails
    pass

The if not result: pass is a no-op - the PATH update below already runs unconditionally. Removed the dead branch and clarified why PATH is updated regardless.

Behavior

No behavior change: both removed branches were unreachable or no-ops.

Testing

ruff check and ruff format --check (v0.15.9, repo-pinned) pass on both changed files.

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[aryanputta]

Note for triage: I can't set labels/milestones as an external contributor. Intended metadata for pr-metadata-check: label cuda.pathfinder, milestone cuda.pathfinder next. Happy to adjust if a different bucket fits better.