fix: use exact Invalid checkpoint token match

Match the backend error prefix case-sensitively and restore the
single-unit test description preferred in review.

Author: vyrnsynx

packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/exceptions.py

@@ -13,7 +13,7 @@
 TOO_MANY_REQUESTS_ERROR: int = 429
 SERVICE_ERROR: int = 500
 INVALID_PARAMETER_VALUE_EXCEPTION: str = "InvalidParameterValueException"
-INVALID_CHECKPOINT_TOKEN_PREFIX: str = "invalid checkpoint token"
+INVALID_CHECKPOINT_TOKEN_PREFIX: str = "Invalid checkpoint token"
 
 # Non-retryable customer error codes that arrive as non-4xx (e.g. HTTP 502) from Lambda.
 # Unlike typical 5xx errors, these require customer intervention (e.g., fixing
@@ -162,7 +162,7 @@ def _classify_error_category(
           These arrive as HTTP 502 but require customer intervention to fix.
         - 4xx errors → EXECUTION, except:
           - 429 (TooManyRequests) → INVOCATION (throttling is transient)
-          - InvalidParameterValueException with "Invalid checkpoint token" (case-insensitive) → INVOCATION
+          - InvalidParameterValueException with "Invalid checkpoint token" (exact match) → INVOCATION
             (stale token from a concurrent checkpoint; next invocation gets a fresh token)
         - 5xx, network errors → INVOCATION
         """
@@ -180,9 +180,9 @@ def _classify_error_category(
             and error
             and not (
                 (error.get("Code") or "") == INVALID_PARAMETER_VALUE_EXCEPTION
-                and (error.get("Message") or "")
-                .casefold()
-                .startswith(INVALID_CHECKPOINT_TOKEN_PREFIX)
+                and (error.get("Message") or "").startswith(
+                    INVALID_CHECKPOINT_TOKEN_PREFIX
+                )
             )
         ):
             return DurableApiErrorCategory.EXECUTION

packages/aws-durable-execution-sdk-python/tests/exceptions_test.py

@@ -65,19 +65,12 @@ def test_checkpoint_error():
     assert error.termination_reason == TerminationReason.CHECKPOINT_FAILED
 
 
-@pytest.mark.parametrize(
-    "message",
-    [
-        "Invalid checkpoint token: token expired",
-        "Invalid Checkpoint Token: token expired",
-    ],
-)
-def test_checkpoint_error_classification_invalid_token_invocation(message: str):
-    """Service emits lowercase 'checkpoint token'; match case-insensitively as invocation."""
+def test_checkpoint_error_classification_invalid_token_invocation():
+    """Test 4xx InvalidParameterValueException with "Invalid checkpoint token" error message is an invocation error."""
     error_response = {
         "Error": {
             "Code": "InvalidParameterValueException",
-            "Message": message,
+            "Message": "Invalid checkpoint token: token expired",
         },
         "ResponseMetadata": {"HTTPStatusCode": 400},
     }