test(fips): make the no-native POST tests genuinely backend-free under -S

The fail-closed tests copy the package, strip every .so, and assert import
refuses crypto. Under an editable install (pip install -e ., as CI runs),
setuptools registers a .pth meta-path finder that maps ama_cryptography.* —
including the compiled Cython bindings — back to the developer build tree, so
the subprocess resolves a working native backend even though the copy under
test has none. The "no native backend" premise was silently false and the
tests were passing for the wrong reason.

Run those subprocesses with -S (skip site processing, so the .pth finder is
never installed). The subprocess then sees only the stripped copy on
PYTHONPATH and the stdlib — a genuinely backend-free tree, which is also what
a real deployment without the C library looks like (the Cython bindings
dynamically link libama_cryptography and cannot load without it). The core
package imports with no third-party dependency (INVARIANT-1), so dropping
site costs these subprocesses nothing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UvSmEE59hFdt3MJHGWzgCJ

Author: claude

tests/test_post_failclosed.py

@@ -62,16 +62,37 @@
 
 
 def _run_python(
-    code: str, cwd: Path, env_extra: dict[str, str] | None = None
+    code: str,
+    cwd: Path,
+    env_extra: dict[str, str] | None = None,
+    *,
+    isolated: bool = False,
 ) -> subprocess.CompletedProcess[str]:
-    """Run ``code`` in a fresh interpreter rooted at ``cwd``."""
+    """Run ``code`` in a fresh interpreter rooted at ``cwd``.
+
+    ``isolated=True`` adds ``-S`` (do not import ``site``). The no-native tests
+    need it. When this test suite runs against an *editable* install
+    (``pip install -e .``, as CI does), setuptools registers a meta-path finder
+    via a ``.pth`` file that maps ``ama_cryptography.*`` — including the compiled
+    Cython bindings — back to the developer's build tree. That finder resolves
+    ``ama_cryptography.sha3_binding`` even when the copy under ``cwd`` has had
+    every ``.so`` removed, so ``native_sha3_256`` finds a working Cython backend
+    and the "no native backend" premise is silently false. ``-S`` skips
+    ``.pth`` processing, so the subprocess sees only the copy on ``PYTHONPATH``
+    (still honoured under ``-S``) and the stdlib — a genuinely backend-free tree,
+    which is also what a real deployment without the C library looks like (the
+    Cython bindings dynamically link ``libama_cryptography`` and cannot load
+    without it). The core package imports with no third-party dependency
+    (INVARIANT-1), so dropping ``site`` costs these subprocesses nothing.
+    """
     env = dict(os.environ)
     env.pop("PYTHONPATH", None)
     # An installed copy of the package would shadow the tree under test.
     env["PYTHONPATH"] = str(cwd)
     env.update(env_extra or {})
+    argv = [sys.executable, "-S", "-c"] if isolated else [sys.executable, "-c"]
     return subprocess.run(
-        [sys.executable, "-c", textwrap.dedent(code)],
+        [*argv, textwrap.dedent(code)],
         cwd=str(cwd),
         env=env,
         capture_output=True,
@@ -126,6 +147,7 @@ def test_missing_native_backend_makes_import_raise(self, tree_without_native: Pa
             print("verified")
             """,
             cwd=tree_without_native,
+            isolated=True,
         )
 
         assert result.returncode != 0, (
@@ -143,7 +165,7 @@ def test_failure_message_names_the_real_cause(self, tree_without_native: Path) -
         ``native Ed25519 not built`` was a claim about the C build, and it was
         usually false: the library was built and simply not on the search path.
         """
-        result = _run_python("import ama_cryptography", cwd=tree_without_native)
+        result = _run_python("import ama_cryptography", cwd=tree_without_native, isolated=True)
 
         combined = result.stdout + result.stderr
         assert "no native library found" in combined
@@ -189,6 +211,7 @@ def test_diagnostic_flag_permits_import_but_not_crypto(self, tree_without_native
                 raise AssertionError("crypto ran in the ERROR state")
             """,
             cwd=tree_without_native,
+            isolated=True,
             env_extra={"AMA_POST_DIAGNOSTIC_IMPORT": "1"},
         )
         assert result.returncode == 0, result.stdout + result.stderr
@@ -914,6 +937,7 @@ def test_no_backend_fails_post_rather_than_skipping(self, tree_without_native: P
             import ama_cryptography  # noqa: F401
             """,
             cwd=tree_without_native,
+            isolated=True,
             env_extra={"AMA_POST_DIAGNOSTIC_IMPORT": "1"},
         )
         assert result.returncode == 0, result.stderr
@@ -942,6 +966,7 @@ def test_docs_build_override_is_honoured(self, tree_without_native: Path) -> Non
                 raise AssertionError("crypto ran under the docs override")
             """,
             cwd=tree_without_native,
+            isolated=True,
             env_extra={"AMA_SPHINX_BUILD": "1"},
         )
         assert result.returncode == 0, result.stdout + result.stderr