Add cloud-based integration tests for filesystem serdes

Filesystem serdes currently has no cloud-based test coverage. All existing tests run locally against mocked filesystem calls, temp directories, or the in-memory runner. We have no test that validates the serdes against a real durable filesystem mount (Amazon S3 Files or EFS) on a deployed Lambda - which is the only environment the feature is actually supported in.

#### Proposed work
- Provision a durable filesystem mount (EFS, and/or S3 Files) for a dedicated example function in the cloud test stack.
- Add a test that uses the cloud durable_runner fixture rather than the local runner, covering ALWAYS, OVERFLOW, preview, and replay/deserialize-from-file end-to-end.

#### Acceptance criteria
- 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 cloud test stack and the cloud durable_runner fixture, then trace the existing filesystem serdes coverage for ALWAYS, OVERFLOW, preview, and replay. Provision the proposed durable filesystem mount for a dedicated example function and run the test against a deployed Lambda. Done means CI verifies write-on-first-invocation and read-on-replay through the checkpoint envelope.

Written by the indexing model from the issue text.