Implement task routing for multi-threaded execution

[AR-May]

For the new multi-threaded execution mode, we need to implement a task routing mechanism. Tasks that support multi-threaded execution should run within the thread node, while those that do not should be routed to the sidecar taskhost process.

For the initial version, the routing decision will rely on an allowlist of MSBuild tasks that are known to be thread-safe. Once #11828 is completed, the decision logic will evolve to detect the appropriate gesture automatically.