.Net Processes - Fix Serialization of Input Data

[crickman]

Motivation and Context

Updated ability to serialize complex types here: #9525

...but missed the remoting boundary for invoke the process. This change addresses this final gap without altering external patterns.

Description

Convert event to JSON when invoking ProcessActor from DaprKernelProcessContext

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄