implement serialization for partition_data in 1d_stencil_5

[the-ivii]

Fixes #6874

Proposed Changes

• i fixed 1d_stencil_5 crash by implementing the missing serialization logic for partition_data,which was previously empty and this ensures the data is correctly packed and transmitted during HPX actions, preventing the system from crashing on invalid empty objects.

Any background context you want to provide?

Checklist

Not all points below apply to all pull requests.

• I have added a new feature and have added tests to go along with it.
• I have fixed a bug and have added a regression test.
• I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

[StellarBot]

Can one of the admins verify this patch?

[hkaiser]

While this is not actually needed for this example, it doesn't hurt to have the serialization implemented as a guideline for people on how to do that.

LGTM, thanks!

[hkaiser]

@the-ivii thank you for working on this!