Fix: runsettings environment variables ignored during test discovery

[Copilot]

During test discovery for DynamicData tests, environment variables defined in .runsettings files were not available to ITestDataSource implementations, causing tests to fail or display incorrect names in Test Explorer.

RunSettingsEnvironmentVariableProvider only checked the --settings command line option, ignoring environment variables used by VSTestBridge to pass runsettings configuration:

• TESTINGPLATFORM_EXPERIMENTAL_VSTEST_RUNSETTINGS (inline XML)
• TESTINGPLATFORM_VSTESTBRIDGE_RUNSETTINGS_FILE (file path)

Changes

• Updated RunSettingsEnvironmentVariableProvider.IsEnabledAsync() to check environment variables in addition to command line option
• Added cascading fallback: command line → inline XML env var → file path env var
• Added comprehensive unit tests covering all runsettings sources

Example

internal class ExampleTestDataSource : Attribute, ITestDataSource
{
    public IEnumerable<object?[]> GetData(MethodInfo methodInfo)
    {
        // This now works during test discovery
        yield return new object[] { Environment.GetEnvironmentVariable("TEST_ENV") };
    }
}

With .runsettings:

<RunSettings>
  <RunConfiguration>
    <EnvironmentVariables>
      <TEST_ENV>TestValue</TEST_ENV>
    </EnvironmentVariables>
  </RunConfiguration>
</RunSettings>

Environment variables are now properly set during discovery, allowing data sources to access them.

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>runsettings are ignored during test discovery (data driven tests)</issue_title>
<issue_description>## Describe the bug

During test discovery of DynamicData tests (using the ITestDataSource), the <EnvironmentVariables> section of the selected .runsettings file is ignored.

Steps To Reproduce

1. Implement the following:

    internal class ExampleTestDataSource : Attribute, ITestDataSource
    {
        public IEnumerable<object?[]> GetData(MethodInfo methodInfo)
        {
            yield return new object[] { Environment.GetEnvironmentVariable("TEST_ENV") };
        }

        public string? GetDisplayName(MethodInfo methodInfo, object?[]? data)
        {
            if (data != null)
            {
                var value = (string)data[0];
                return $"{methodInfo.Name} - {value}";
            }
            return null;
        }
    }

    [TestClass]
    public class MyTests
    {
        [TestMethod]
        [ExampleTestDataSource]
        public void MyTest(string name)
        {
            Assert.IsTrue(!string.IsNullOrEmpty(name));
        }
    }

2. Create a .runnsettings file with the following section:

<RunSettings>
	<RunConfiguration>
		<EnvironmentVariables>
			<TEST_ENV>This is a string</TEST_ENV>
		</EnvironmentVariables>
	</RunConfiguration>
</RunSettings>

3. Configure TestExplorer to use your .runsettings file.

4. Observe that the test name (in test explorer) does not include the "This is a string" token.

5. Observe that MyTest fails.

Expected behavior

.Runsettings to be used during test discovery.

Actual behavior

.runsettings is ignored during test discovery.

</issue_description>

Comments on the Issue (you are @copilot in this section)

@Youssef1313 The issue here is in VSTestBridge. The `RunSettingsEnvironmentVariableProvider` implementation doesn't consider `TESTINGPLATFORM_EXPERIMENTAL_VSTEST_RUNSETTINGS`.

• Fixes runsettings are ignored during test discovery (data driven tests) #6990

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