[messages] datetime is incorrectly formatted in dialogs API

[YUCLing]

Fixes #0000

Changes proposed in this pull request:
Add correct casts to Dialog and UserDialogState so that the datetime from database is converted to Carbon instead of string.

Without casting, the time displayed on the frontend will be the UTC time treated as local time.

Reviewers should focus on:

Screenshot
Before

After

QA

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)