Add underscore separator support to TestTree for test name hierarchies

[Copilot]

TestTree only split test names on dots (.), preventing proper hierarchical organization of tests using underscore-based naming conventions like Method_WhenCondition_ThenResult.

Changes

• TestTree.cs: Added '_' to NameSeparators array alongside '.'
• TestTreeTests.cs: Added tests verifying underscore separation behavior

Example

var tree = new TestTree();
tree.AddTests(["Namespace.Class.Method_WhenCondition_ThenResult"]);

// Before: Single node "Method_WhenCondition_ThenResult"
// After:  Hierarchy Method → WhenCondition → ThenResult

var node = tree.FindNodeByPath("Namespace.Class.Method");
Assert.Equal("Method", node.Name);  // Now passes

This enables TestTree to handle common test naming patterns (AAA/GWT) without breaking existing dot-separated hierarchies.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Failing Test: Asynkron.TestRunner.Tests.TestTreeTests.FindNodeByPath_WorksWithUnderscoreSeparators</issue_title>
<issue_description>## Failing Test

Test: Asynkron.TestRunner.Tests.TestTreeTests.FindNodeByPath_WorksWithUnderscoreSeparators
Display Name: Asynkron.TestRunner.Tests.TestTreeTests.FindNodeByPath_WorksWithUnderscoreSeparators

Error Message

Assert.Equal() Failure: Strings differ
Expected: "Method"
Actual:   "Method_WhenCondition_ThenResult"
                 ↑ (pos 6)

Stack Trace

   at Xunit.Assert.Equal(ReadOnlySpan`1 expectedSpan, ReadOnlySpan`1 actualSpan, Boolean ignoreCase, Boolean ignoreLineEndingDifferences, Boolean ignoreWhiteSpaceDifferences, Boolean ignoreAllWhiteSpace) in /_/src/xunit.assert/Asserts/SpanAsserts.cs:line 617
   at Xunit.Assert.Equal(String expected, String actual, Boolean ignoreCase, Boolean ignoreLineEndingDifferences, Boolean ignoreWhiteSpaceDifferences, Boolean ignoreAllWhiteSpace) in /_/src/xunit.assert/Asserts/StringAsserts.cs:line 333
   at Xunit.Assert.Equal(String expected, String actual) in /_/src/xunit.assert/Asserts/StringAsserts.cs:line 302
   at Asynkron.TestRunner.Tests.TestTreeTests.FindNodeByPath_WorksWithUnderscoreSeparators() in /Users/rogerjohansson/git/asynkron/Asynkron.TestRunner/tests/Asynkron.TestRunner.Tests/TestTreeTests.cs:line 243
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Other Failing Tests in This Run

• Asynkron.TestRunner.Tests.TestTreeTests.AddTests_TestsWithUnderscoreSeparators_SplitsCorrectly

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Generated by Asynkron.TestRunner at 2026-01-16 11:41:23 UTC
</issue_description>

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

• Fixes Failing Test: Asynkron.TestRunner.Tests.TestTreeTests.FindNodeByPath_WorksWithUnderscoreSeparators #10

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