Remove APM use outside of TaskHost node provider / endpoint

[ccastanedaucf]

Switches preprocessor directives to allow .NET Framework to use TPL-based concurrency over the old APM model. Since MSBuildClientPacketPump and NodeProviderOutOfProcBase never run on the TaskHost, this allows removing a chunk of (now) dead code.

(3rd part of re-attempting #11546)

[Copilot]

Pull Request Overview

This PR removes legacy APM code paths from non‐TaskHost builds in favor of TPL‐based concurrency while retaining APM only when running on a TaskHost. Key changes include adjusting preprocessor directives from FEATURE_APM/NET451_OR_GREATER to TASKHOST, removing large blocks of obsolete APM code, and updating asynchronous read logic for consistency.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Shared/NodeEndpointOutOfProcBase.cs	Updated preprocessor directives and logic to use TPL-based concurrency when not in TaskHost, and switch to APM only in TaskHost scenarios.
src/Shared/CommunicationsUtilities.cs	Modified async read method conditions to align with the new TASKHOST flag and .NET conditional compilation patterns.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs	Removed legacy APM handling methods and integrated minimal logging for NETFRAMEWORK in the packet read loop.
src/Build/BackEnd/Client/MSBuildClientPacketPump.cs	Eliminated FEATURE_APM branches in favor of TPL-based reads for non-TaskHost configurations.

Comments suppressed due to low confidence (3)

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs:634

• [nitpick] The removal of the FEATURE_APM branch here is appropriate; please confirm that all TaskHost-specific scenarios are covered and legacy APM code is no longer expected in other execution contexts.

public void BeginAsyncPacketRead() { ... }

src/Build/BackEnd/Client/MSBuildClientPacketPump.cs:204

• [nitpick] Ensure that the removal of FEATURE_APM branches here is coordinated with the overall TASKHOST strategy so that TPL-based concurrency is seamlessly adopted in non-TaskHost builds.

Task<int> readTask = CommunicationsUtilities.ReadAsync(localStream, headerByte, headerByte.Length).AsTask();

src/Shared/CommunicationsUtilities.cs:589

• [nitpick] Verify that switching the condition to '!TASKHOST' correctly excludes TaskHost builds from using the TPL-based ReadAsync logic, aligning with the overall design.

#if !TASKHOST

[Copilot]

[nitpick] Consider adding an inline comment here to clarify that '!TASKHOST' triggers TPL-based concurrency and that APM logic is reserved for TaskHost builds.

Suggested change

	#endif
	#endif
	// The '!TASKHOST' directive triggers TPL-based concurrency by including System.Threading.Tasks.
	// APM (Asynchronous Programming Model) logic is reserved for TaskHost builds.

[Copilot]

[nitpick] Ensure that TASKHOST is defined only for TaskHost scenarios since it now controls the use of legacy APM code in this section.

Suggested change

	// Wait for a connection
	// Wait for a connection
	// Ensure TASKHOST is defined only for TaskHost scenarios, as it controls the use of legacy APM code.

[SimaTian]

Looks good to me. Removing ancient way of doing stuff in favor of the more modern one.
Just to be extra safe (since we had some issues before) I will run our battery of perf tests.