Report actual launch path in MSB4216 for Runtime="NET" task host

[ViktorHofer]

Fixes #13888

Summary

When a Runtime="NET" task host launch fails, the MSB4216 error message
always pointed at <sdk>\MSBuild.exe — the .NET task host app host — even
when the launcher fell back to dotnet.exe MSBuild.dll because no app host
was present in the SDK.

SDK 10.0.x ships MSBuild.dll only; the app host first appears in 10.0.300.
On those SDKs users see an MSB4216 referencing a file that never existed
(see #13879 for a concrete repro on Windows-ARM with SDK 10.0.108).

Change

Centralize the apphost-vs-fallback decision in a new
NodeProviderOutOfProcTaskHost.ResolveNetTaskHostLaunchPath helper that
returns (LaunchPath, UseAppHost). Use it from both the launcher
(ResolveAppHostOrFallback) and the error path
(TaskHostTask.LogErrorUnableToCreateTaskHost) so the reported path
always matches the path that was actually launched.

This is a logging-only change; launch behavior is unchanged.

Notes

This is a partial fix for #13879. The underlying handshake bug
(IsAllowedBitnessMismatch missing Arm64 tolerance) remains and will be
fixed separately.

[Copilot]

Pull request overview

This PR updates MSBuild’s out-of-proc .NET task host handling so MSB4216 reports the actual launch target path for Runtime="NET" task hosts (app host vs dotnet-hosted MSBuild.dll), aligning user-facing diagnostics with the launcher’s resolution logic.

Changes:

• Centralizes apphost-vs-fallback selection in NodeProviderOutOfProcTaskHost.ResolveNetTaskHostLaunchPath.
• Uses the shared resolver in both the launcher (ResolveAppHostOrFallback) and MSB4216 error construction (TaskHostTask.LogErrorUnableToCreateTaskHost).
• Updates trace output and fallback command line to consistently use the resolved launch path.

Show a summary per file

File	Description
src/Build/Instance/TaskFactories/TaskHostTask.cs	Uses the shared resolver to report the resolved NET task host launch path in MSB4216 instead of always pointing at the app host.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs	Introduces ResolveNetTaskHostLaunchPath and updates launcher logic to use the resolved app host or MSBuild.dll fallback consistently.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 3

[github-actions]

Note

🔒 Integrity filter blocked 2 items

The following items were blocked because they don't meet the GitHub integrity level.

• #13889 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #13889 pull_request_read: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

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Generated by Expert Code Review (on open) for issue #13889 · ● 20.3M

[ViktorHofer]

@rainersigwald @JanProvaznik would appreciate your review on this one. Straightforward minimal change to log the actual invocation instead of always the apphost.