test(golden): parametrized corpus tests with actionable hints

One parametrized test per case for build + package output. Mismatch
hint points at update_goldens.py and explains the semver gate's
expectations.

Author: bnusunny

tests/golden/conftest.py

@@ -0,0 +1,21 @@
+"""Pytest fixtures for the golden-template corpus.
+
+Each case directory under tests/golden/templates/ becomes one parametrized
+test ID (relative path), enabling `pytest -k <name>` selection.
+"""
+
+from pathlib import Path
+
+import pytest
+
+TEMPLATES_ROOT = Path(__file__).parent / "templates"
+
+
+def pytest_generate_tests(metafunc):
+    if "golden_case" in metafunc.fixturenames:
+        cases = sorted(p.parent for p in TEMPLATES_ROOT.rglob("template.yaml"))
+        metafunc.parametrize(
+            "golden_case",
+            cases,
+            ids=lambda p: str(p.relative_to(TEMPLATES_ROOT)),
+        )

tests/golden/test_corpus.py

@@ -0,0 +1,48 @@
+"""Golden corpus: byte-exact diff for build + package output per case."""
+
+from pathlib import Path
+
+import pytest
+
+from tests.golden.harness import run_build_pipeline, run_package_pipeline
+from tests.golden.normalize import normalize
+from tests.golden.update_goldens import _read_metadata, _resolve_le_default
+
+TEMPLATES_ROOT = Path(__file__).parent / "templates"
+
+
+def _hint(case_dir: Path) -> str:
+    rel = case_dir.relative_to(TEMPLATES_ROOT)
+    return (
+        f"\nGolden mismatch in {rel}.\n"
+        f"To inspect:   python tests/golden/update_goldens.py --diff --filter '{rel}'\n"
+        f"To re-pin:    python tests/golden/update_goldens.py --filter '{rel}'\n"
+        f"If intentional:\n"
+        f"  - new case (added expected.*.yaml)         -> no version bump at PR time\n"
+        f"  - modified/deleted existing expected.*     -> bump major in samcli/__init__.py\n"
+    )
+
+
+def test_build_output_matches_golden(golden_case):
+    meta = _read_metadata(golden_case)
+    le_enabled = _resolve_le_default(golden_case, meta)
+    actual = normalize(
+        run_build_pipeline(golden_case / "template.yaml", language_extensions=le_enabled)
+    )
+    expected_path = golden_case / "expected.build.yaml"
+    assert expected_path.exists(), f"missing {expected_path}; run update_goldens.py --new"
+    expected = expected_path.read_text(encoding="utf-8")
+    assert actual == expected, _hint(golden_case)
+
+
+def test_package_output_matches_golden(golden_case):
+    meta = _read_metadata(golden_case)
+    le_enabled = _resolve_le_default(golden_case, meta)
+    build_out = run_build_pipeline(
+        golden_case / "template.yaml", language_extensions=le_enabled
+    )
+    actual = normalize(run_package_pipeline(golden_case / "template.yaml", build_out))
+    expected_path = golden_case / "expected.package.yaml"
+    assert expected_path.exists(), f"missing {expected_path}; run update_goldens.py --new"
+    expected = expected_path.read_text(encoding="utf-8")
+    assert actual == expected, _hint(golden_case)