[Bug]: Invalid checkpoint token misclassified as execution failure (message case mismatch)

### Expected Behavior

When the service rejects a checkpoint call with `InvalidParameterValueException` because the checkpoint token is stale (a newer invocation superseded this one), the SDK should classify the error as an **invocation** failure (retryable). The current invocation ends, and the service starts a fresh invocation with a fresh token. The execution itself continues.

### Actual Behavior

The SDK classifies the error as an **execution** failure (non-retryable). So a stale checkpoint token fails the whole durable execution every time.

### Root Cause

The classifier matches the error message with a case-sensitive prefix check against `"Invalid Checkpoint Token"`. The service emits `"Invalid checkpoint token"` (lowercase `c` and `t`). The prefix check never matches. The error falls through to the generic 4xx branch, which is classified as an execution failure.

Service constant (`DurableExecutionsServiceSharedLib`, `ValidationErrorMessages.java` line 86, internal):

```java
public static final String INVALID_CHECKPOINT_TOKEN = "Invalid checkpoint token";
```

SDK constant (`packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/exceptions.py`):
https://github.com/aws/aws-durable-execution-sdk-python/blob/8742ad9c9535e1de042963963c6b439f656894ce/packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/exceptions.py#L16

```python
INVALID_CHECKPOINT_TOKEN_PREFIX: str = "Invalid Checkpoint Token"
```

Used by `_classify_error_category()` via `(error.get("Message") or "").startswith(INVALID_CHECKPOINT_TOKEN_PREFIX)`.

Test that encodes the wrong string: `tests/exceptions_test.py` (`"Invalid Checkpoint Token: token expired"`).

### Steps to Reproduce

1. Have the service return a 4xx `InvalidParameterValueException` whose message starts with `Invalid checkpoint token` (the real service message).
2. Pass it through the SDK error classifier.
3. Observe it is classified as an execution (non-retryable) error rather than an invocation (retryable) error.

The existing unit tests do not catch this. They construct the error with the message `"Invalid Checkpoint Token: token expired"`, which matches the SDK constant rather than the service constant.

### Proposed Fix

1. Change the SDK constant to `"Invalid checkpoint token"` so it matches the service.
2. Consider a case-insensitive comparison to protect against future drift.
3. Update the unit tests to use the real service message.

### Related

Same defect exists in all three SDKs; each gets its own issue.
- JS: aws/aws-durable-execution-sdk-js
- Python: aws/aws-durable-execution-sdk-python
- Java: aws/aws-durable-execution-sdk-java

### SDK Version

main (8742ad9)