Fix assignment of typed search attributes on CaN and child workflows

[THardy98]

What was changed

A regression was introduced in this change that directly assigned typed search attributes to SearchAttributes.IndexedFields on continue-as-new or for child workflows.

However, SearchAttributes is null on this assignment, resulting in a null assignment reference:

18:50:28 warn: Temporalio.Workflow:kitchenSink[0] => System.Collections.Generic.Dictionary`2[System.String,System.Object] => System.Collections.Generic.Dictionary`2[System.String,System.Object] Failed activation on workflow kitchenSink with ID w-dotnet-20260331-183334-8b9e-1f62d5c4b00f1126-8 and run ID 019d453b-958a-7f2a-87bf-6711fad00bca System.NullReferenceException: Object reference not set to an instance of an object.    at Temporalio.Worker.WorkflowInstance.OutboundImpl.StartChildWorkflowAsync[TWorkflow,TResult](StartChildWorkflowInput input) in /app/repo/src/Temporalio/Worker/WorkflowInstance.cs:line 2423
...

This change fixed the assignment and adds a couple small integration tests to verify the fix.

2. How was this tested:
   Couple integration tests

3. Any docs updates needed?
   Maybe ?