test: add tests for cache eviction and narrowed exception handling

- test_cache_evicts_oldest_when_full: verifies FIFO eviction at _MAX_CACHE_SIZE
- test_export_cloudformation_stack_unexpected_exception_propagates: verifies
  unexpected exceptions (TypeError etc.) are not silently swallowed
- Updated existing expansion_failure test to use InvalidSamDocumentException

Author: bnusunny

tests/unit/lib/cfn_language_extensions/test_sam_integration.py

@@ -785,6 +785,7 @@ def test_frozen(self):
 
 from samcli.lib.cfn_language_extensions.sam_integration import (
     _expansion_cache,
+    _MAX_CACHE_SIZE,
     clear_expansion_cache,
 )
 
@@ -973,3 +974,38 @@ def test_non_language_ext_template_cached(self):
             assert len(_expansion_cache) == 1
             # Ensure independence
             assert result1.expanded_template is not result2.expanded_template
+
+    def test_cache_evicts_oldest_when_full(self):
+        """Cache should evict the oldest entry when _MAX_CACHE_SIZE is reached."""
+        clear_expansion_cache()
+        template = {
+            "Transform": "AWS::LanguageExtensions",
+            "Resources": {
+                "Fn::ForEach::Loop": [
+                    "Item",
+                    ["A"],
+                    {"Res${Item}": {"Type": "AWS::SNS::Topic"}},
+                ]
+            },
+        }
+
+        with tempfile.TemporaryDirectory() as tmp:
+            paths = []
+            for i in range(_MAX_CACHE_SIZE + 1):
+                path = os.path.join(tmp, f"template_{i}.yaml")
+                with open(path, "w") as f:
+                    f.write("AWSTemplateFormatVersion: '2010-09-09'\n")
+                paths.append(path)
+
+            # Fill the cache to capacity
+            for i in range(_MAX_CACHE_SIZE):
+                expand_language_extensions(template, template_path=paths[i])
+
+            assert len(_expansion_cache) == _MAX_CACHE_SIZE
+            first_key = next(iter(_expansion_cache))
+
+            # One more should evict the oldest
+            expand_language_extensions(template, template_path=paths[_MAX_CACHE_SIZE])
+
+            assert len(_expansion_cache) == _MAX_CACHE_SIZE
+            assert first_key not in _expansion_cache

tests/unit/lib/package/test_artifact_exporter.py

@@ -15,6 +15,7 @@
 from samcli.commands._utils.experimental import ExperimentalFlag
 from samcli.commands.package import exceptions
 from samcli.commands.package.exceptions import ExportFailedError
+from samcli.commands.validate.lib.exceptions import InvalidSamDocumentException
 from samcli.lib.package.artifact_exporter import (
     is_local_folder,
     make_abs_path,
@@ -2515,7 +2516,7 @@ def test_export_cloudformation_stack_language_extensions_expansion_failure(self,
 
             with patch(
                 "samcli.lib.cfn_language_extensions.sam_integration.expand_language_extensions",
-                side_effect=Exception("expansion failed"),
+                side_effect=InvalidSamDocumentException("expansion failed"),
             ):
                 stack_resource.export(resource_id, resource_dict, parent_dir)
 
@@ -2526,3 +2527,32 @@ def test_export_cloudformation_stack_language_extensions_expansion_failure(self,
             self.assertNotIn("template_str", call_kwargs.kwargs)
         finally:
             os.remove(template_path)
+
+    @patch("samcli.lib.package.artifact_exporter.Template")
+    def test_export_cloudformation_stack_unexpected_exception_propagates(self, TemplateMock):
+        """
+        When expand_language_extensions raises an unexpected exception (not InvalidSamDocumentException),
+        it should propagate instead of being silently swallowed.
+        """
+        stack_resource = CloudFormationStackResource(self.uploaders_mock, self.code_signer_mock)
+
+        resource_id = "NestedStack"
+        property_name = stack_resource.PROPERTY_NAME
+
+        handle = tempfile.NamedTemporaryFile(suffix=".yaml", mode="w", delete=False)
+        try:
+            handle.write("Resources: {}")
+            handle.flush()
+            template_path = handle.name
+            handle.close()
+            resource_dict = {property_name: template_path}
+            parent_dir = tempfile.gettempdir()
+
+            with patch(
+                "samcli.lib.cfn_language_extensions.sam_integration.expand_language_extensions",
+                side_effect=TypeError("unexpected bug"),
+            ):
+                with self.assertRaises(ExportFailedError):
+                    stack_resource.export(resource_id, resource_dict, parent_dir)
+        finally:
+            os.remove(template_path)