Draft: Executor serialization follow-up

[ShiyunXu]

Motivation & Context

This draft PR tracks the executor serialization change that was described in PR microsoft#6776 and preserves the workflow behavior for serializing deliveries to the same executor within a superstep.

Description & Review Guide

• What are the major changes?
  • Capture the executor-serialization behavior under a draft PR so the work can be reviewed and discussed.
• What is the impact of these changes?
  • No functional code changes are introduced in this draft; it is a tracking PR for the executor serialization behavior already discussed for PR Python: Process messages to an executor serially within a superstep microsoft/agent-framework#6776.
• What do you want reviewers to focus on?
  • Confirm the draft scope and whether a follow-up implementation PR is still needed.

Related Issue

Fixes microsoft#6776

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and the title prefix in sync automatically.