Report PID-qualified testhost controller pipe env var

[Copilot]

When the testhost controller pipe-name lookup failed, the error reported the base environment variable instead of the PID-qualified key that was actually read. This made startup diagnostics point at the wrong variable.

• Diagnostic fix
  • Report the computed TESTINGPLATFORM_TESTHOSTCONTROLLER_PIPENAME_<pid> key in the missing pipe-name exception.

string pipeEnvironmentVariable =
    $"{EnvironmentVariableConstants.TESTINGPLATFORM_TESTHOSTCONTROLLER_PIPENAME}_{testHostControllerInfo.GetTestHostControllerPID()}";

string pipeName = environment.GetEnvironmentVariable(pipeEnvironmentVariable)
    ?? throw new InvalidOperationException(
        $"Unexpected null pipe name from environment variable '{pipeEnvironmentVariable}'");

• Regression coverage
  • Added a focused unit test that verifies the exception message includes the PID-qualified environment variable name.

[Copilot]

Pull request overview

Fixes a startup diagnostic in Microsoft.Testing.Platform where a missing testhost-controller pipe-name error reported the wrong environment variable key, and adds a unit test to prevent regressions.

Changes:

• Update the thrown exception message to include the PID-qualified environment variable name that was actually read.
• Add a unit test asserting the exception message contains the PID-qualified key when the pipe name is missing.

Show a summary per file

File	Description
test/UnitTests/Microsoft.Testing.Platform.UnitTests/TestHostBuilderTests.cs	Adds regression test verifying the PID-qualified env-var name is present in the thrown exception message.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.Utilities.cs	Fixes the exception message to report the PID-qualified pipe env-var key instead of the base constant.

Copilot's findings

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

[Evangelink]

@copilot address review comments

[Copilot]

@copilot address review comments

Addressed in c0f71cb. The test now fails clearly if the reflected method cannot be found and unwraps synchronous TargetInvocationException from MethodInfo.Invoke while preserving the original exception stack.