Fix NullReferenceException in WorkflowHost with .NET 8+ dependency injection

[Copilot]

Describe the change
Fixes NullReferenceException occurring when WorkflowHost starts on .NET 8+ with newer Microsoft.Extensions.DependencyInjection versions (8.0+, 9.0+, 10.0+).

Describe your implementation or design
Two separate issues were causing NullReferenceException:

1. DI Registration Issue: Using GetService<T>() instead of GetRequiredService<T>() when registering ILifeCycleEventPublisher as an IBackgroundTask meant that if the service failed to resolve, null would be added to the IEnumerable<IBackgroundTask> collection, causing a NullReferenceException when iterating.

2. Activity Tracing Issue: When OpenTelemetry tracing is not enabled, WorkflowActivity.StartHost() returns null. The exception handler in StartAsync was calling activity.AddException(ex) without checking for null, causing a NullReferenceException.

Changes:

• ServiceCollectionExtensions.cs: Changed sp.GetService<ILifeCycleEventPublisher>() to sp.GetRequiredService<ILifeCycleEventPublisher>() for fail-fast behavior with clear error messages
• WorkflowHost.cs: Added null-safety checks when iterating background tasks in StartAsync() and StopAsync() as defensive programming
• WorkflowHost.cs: Changed activity.AddException(ex) to activity?.AddException(ex) to handle null activity when OpenTelemetry tracing is disabled

Tests
Verified with .NET 10 runtime and Microsoft.Extensions.DependencyInjection 10.0.0:

• Tested with SQL Server persistence provider (WorkflowCore.Persistence.SqlServer)
• All 5 background tasks resolve correctly (WorkflowConsumer, EventConsumer, IndexConsumer, RunnablePoller, LifeCycleEventPublisher)
• Host start/stop operations complete without NullReferenceException
• No null tasks in collection
• Expected SqlException for SQL connection (test environment has no actual SQL Server)

Breaking change
No. If ILifeCycleEventPublisher cannot be resolved (which shouldn't happen in valid configurations), the code now throws a clear exception during service registration instead of a cryptic runtime error—this is an improvement in error reporting.

Additional context
The fail-fast approach provides better diagnostics for misconfigured DI containers while the null checks provide defense-in-depth. The activity null-check ensures compatibility whether OpenTelemetry tracing is enabled or not.

• Fixes NET 10: Object reference not set to an instance of an object #1403

Original prompt

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This section details on the original issue you should resolve

<issue_title>NET 10: Object reference not set to an instance of an object</issue_title>
<issue_description>Describe the bug
I am using Workflowcore with persistance to MS SQL database. Everything worked until I upgraded to NET 10. Now I get error when app is bootstrapping:
Object reference not set to an instance of an object
at WorkflowCore.Services.WorkflowHost.d__44.MoveNext()

If I use in-memory persistance it works. Guess is something with UseSqlServer extension for Workflowcore

To Reproduce
Upgrade to NET 10 and use MS SQL server as persistance</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes NET 10: Object reference not set to an instance of an object #1403

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[danielgerlag]

@copilot Did you test with actual .net 10 and the SQL persistence provider?

[Copilot]

@copilot Did you test with actual .net 10 and the SQL persistence provider?

Yes, I've now tested with actual .NET 10 and SQL Server persistence provider.

The test revealed an additional NullReferenceException that occurred when OpenTelemetry tracing is not enabled - WorkflowActivity.StartHost() returns null, and the exception handler was calling activity.AddException(ex) without null-checking.

Fixed in commit e36b2ad by adding the null-conditional operator: activity?.AddException(ex).

Test results with .NET 10 + SQL Server persistence:

• ✅ All 5 background tasks registered correctly (no nulls)
• ✅ WorkflowHost created successfully
• ✅ No NullReferenceException during startup
• ✅ Expected SqlException for connection (no SQL Server available in test environment)

[kosmakoff]

Hi @danielgerlag
I've just stumbled at this exact error. Are you planning to merge this PR anytime soon? Also, what are you plans regarding .NET 10 upgrade? I could help with PR if needed.