Add cloud-based integration tests for filesystem serdes

Filesystem serdes in the JS SDK has no cloud-based test coverage, and no example at all. The only test is a co-located unit test that fully mocks the filesystem. Nothing validates the serdes against a real durable filesystem mount (Amazon S3 Files or EFS) on a deployed Lambda - the only environment where the feature is actually supported.

#### Proposed work
- Add a filesystem serdes example under packages/aws-durable-execution-sdk-js-examples/src/examples/ and register it in examples-catalog.ts.
- Add an integration test using the CloudDurableTestRunner covering ALWAYS, OVERFLOW, preview, and replay/deserialize-from-file end-to-end.

#### Acceptance criteria
- A filesystem serdes example exists in the examples package and catalog.
- CI runs at least one filesystem serdes test against a deployed Lambda backed by a real durable filesystem mount.
- The test exercises write-on-first-invocation and read-on-replay through the checkpoint envelope.

Start with the co-located filesystem serdes unit test and the CloudDurableTestRunner entry point. Add the example under packages/aws-durable-execution-sdk-js-examples/src/examples/ and register it in examples-catalog.ts, then run the integration coverage against a deployed Lambda with a durable filesystem mount. Done means CI covers ALWAYS, OVERFLOW, preview, replay, and deserialize-from-file through the checkpoint envelope.

Written by the indexing model from the issue text.