fix(sync): Handle timezone-naive datetimes in sync.toml (#8477)

[vicheey]

Which issue(s) does this change fix?

#8477, #8679

Why is this change necessary?

sam sync --watch crashes with TypeError: can't subtract offset-naive and offset-aware datetimes when the .aws-sam/sync.toml file contains timestamps without timezone information (missing +00:00 suffix). This breaks the watch functionality for users who have previously run sync commands, preventing them from using the auto-sync feature.

How does it address the issue?

The fix ensures all timestamps loaded from sync.toml are timezone-aware UTC by checking if tzinfo is None after parsing with datetime.fromisoformat() and adding UTC timezone if missing. This maintains consistency with how SAM CLI writes timestamps (always UTC timezone-aware) and handles both old sync.toml files (without timezone info) and new ones (with timezone info) correctly. The fix is applied in two locations in samcli/commands/sync/sync_context.py:

1. When loading latest_infra_sync_time from the sync state
2. When loading sync_time for each resource sync state

What side effects does this change have?

None. The change is backward compatible and only affects the internal datetime handling. All timestamps are now consistently timezone-aware UTC, which is the expected behavior throughout the sync module.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.