fix: convert InvocationCompletedDetails to unix milliseconds

[yaythomas]

Description of changes:
SQLite storage was failing with "Object of type datetime is not JSON serializable" when attempting to save execution state after Lambda invocation. The root cause was InvocationCompletedDetails.to_dict() returning raw datetime objects instead of JSON-serializable integers.

This fix adds to_json_dict() and from_json_dict() methods to InvocationCompletedDetails that convert datetime objects to/from Unix milliseconds using TimestampConverter, matching the pattern already used by the SDK's Operation class.

This fix the breakage in sam cli.

Error:    DurableFunctionsTestError: Unexpected error during Lambda invocation: None is not a valid InvocationStatus
which is caused by:
...
Error:  ExecutionError: Unexpected payload provided to start the durable execution. Check your resource configurations to confirm the durability is set.
Traceback (most recent call last):
  File "/var/task/aws_durable_execution_sdk_python/execution.py", line 269, in wrapper
    raise ExecutionError(msg) from e

Changes:

• Add InvocationCompletedDetails.to_json_dict() for serialization
• Add InvocationCompletedDetails.from_json_dict() for deserialization
• Update Execution.to_json_dict() to call completion.to_json_dict()
• Update Execution.from_json_dict() to call from_json_dict()

The to_dict() method is preserved for internal use where datetime objects are needed, while to_json_dict() is used for storage and JSON serialization paths.

Fixes execution persistence failures in SQLite and filesystem stores.

Issue #, if available:
closes #193

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[bchampp]

🤖 Emulator PR Created

A draft PR has been created with locked dependencies:

➡️ https://github.com/aws/aws-durable-execution-emulator/pull/324

The emulator will build binaries using the exact testing SDK commit locked in uv.lock.

[bchampp]

🔄 Emulator PR Updated

The emulator PR has been updated with locked dependencies:

➡️ https://github.com/aws/aws-durable-execution-emulator/pull/324

[bchampp]

🔄 Emulator PR Updated

The emulator PR has been updated with locked dependencies:

➡️ https://github.com/aws/aws-durable-execution-emulator/pull/324