Reconsider Task.Run usage for OnTaskCreated and OnTaskUpdated

See this AI comment - https://github.com/a2aproject/a2a-dotnet/pull/70#discussion_r2219135440 for more details

Review the linked AI comment in PR #70 to understand the context around Task.Run usage in OnTaskCreated and OnTaskUpdated handlers. Locate these methods in the codebase, likely in a service or event-handling class. Assess the current threading model and determine if the Task.Run calls are necessary or if they should be replaced with more appropriate async patterns. Testing changes will require understanding the SDK's concurrency behavior.

索引モデルが issue の本文から書いたものです。