Fix TraceEngine file contention deadlock in multithreaded mode

[JanProvaznik]

Problem

In multithreaded (-mt) mode, multiple in-proc node engines run in the same process. Each engine's TraceEngine() method (enabled by MSBUILDDEBUGSCHEDULER=1) wrote to the same EngineTrace_{PID}.txt file with FileShare.Read. When two engines traced concurrently, the second got an IOException, which fatally crashed the ActionBlock work queue. All subsequent Post() calls were silently rejected, causing request completions to be dropped and the scheduler to deadlock.

Root Cause

BuildRequestEngine.TraceEngine() -> FileUtilities.OpenWrite(FileShare.Read) -> IOException -> QueueAction catch handler rethrows -> ActionBlock faults permanently -> subsequent state-change posts silently rejected -> request completion lost -> deadlock

The lock(this) in TraceEngine() only synchronizes within the same engine instance. In -mt mode each in-proc node has its own engine, so the lock provides no cross-engine protection.

Fix

1. Per-node trace files: EngineTrace_{PID}_{NodeId}.txt eliminates file contention entirely
2. IOException catch: Trace IO failures are caught and silently ignored, debug tracing must never crash the build engine

Test

Added MultiThreadedBuild_WithDebugSchedulerTracing_DoesNotDeadlock regression test that sets MSBUILDDEBUGSCHEDULER=1 and builds a root project with two child projects in parallel using MultiThreaded=true. Verifies the build completes successfully (previously deadlocked).