Update test-py command and fix broken test (#521)

* Update test-py command and fix broken test

* Use existing ensure_utc util function

Author: jonathan343

Makefile

@@ -34,7 +34,14 @@ check-py: ## Runs checks (formatting, lints, type-checking) on the python packag
 
 
 test-py: ## Runs tests for the python packages.
-	uv run pytest packages
+    # NOTE: Due to namespace collisions with identically named test files across packages,
+    # run tests per package: uv run pytest packages/package-name/tests
+	@for pkg in packages/*/; do \
+		if [ -d "$$pkg/tests" ]; then \
+			echo "Running tests for $$pkg"; \
+			uv run pytest "$$pkg/tests" || exit 1; \
+		fi; \
+	done
 
 
 build-py: ## Builds the python packages.

packages/smithy-core/src/smithy_core/interfaces/identity.py

@@ -3,6 +3,8 @@
 from datetime import UTC, datetime
 from typing import Protocol, runtime_checkable
 
+from ..utils import ensure_utc
+
 
 @runtime_checkable
 class Identity(Protocol):
@@ -14,6 +16,10 @@ class Identity(Protocol):
     If time zone is provided, it is updated to UTC. The value must always be in UTC.
     """
 
+    def __post_init__(self) -> None:
+        if self.expiration is not None:
+            self.expiration = ensure_utc(self.expiration)
+
     @property
     def is_expired(self) -> bool:
         """Whether the identity is expired."""