Reduce and tighten comments and docstrings across the test suite (#6429)

* Reduce and tighten comments and docstrings in tests

Shorten verbose comments and docstrings across the test suite without
changing any test logic. Remove narration that restates the next line,
collapse long module and test docstrings to a single line, and drop banner
separators. Keep regression context (issue and PR references, run ids),
skip reasons, mocking and timing rationale, license headers, lint and type
directives, and commented-out code.

Comments and docstrings only: an AST signature check confirms no code,
assertions, or string literals changed, and the suite byte-compiles cleanly.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/python/test_cross_platform_parity.py

@@ -13,11 +13,7 @@ INSTALL_PS1 = REPO_ROOT / "install.ps1"
 
 
 class TestNoTorchBackendAutoInInstallSh:
-    """install.sh primary install paths must not use --torch-backend=auto.
-
-    The fallback else-branch (when TORCH_INDEX_URL is empty) is allowed to
-    use --torch-backend=auto since that is the last-resort recovery path.
-    """
+    """install.sh primary paths must not use --torch-backend=auto (only the fallback else-branch may)."""
 
     def test_no_torch_backend_auto_outside_fallback(self):
         lines = INSTALL_SH.read_text(encoding = "utf-8").splitlines()