Harden rewind test path construction

Use Path.Join so a rooted file name cannot discard the E2E working directory.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 91a7e2ab-88a5-4365-accf-cbea1e1391e9

Author: stephentoub

dotnet/test/E2E/RewindE2ETests.cs

@@ -18,7 +18,7 @@ public class RewindE2ETests(E2ETestFixture fixture, ITestOutputHelper output)
     [Fact]
     public async Task Should_Restore_Tracked_File_And_Conversation()
     {
-        var filePath = Path.Combine(Ctx.WorkDir, FileName);
+        var filePath = Path.Join(Ctx.WorkDir, FileName);
         await using var session = await CreateSessionAsync(new SessionConfig
         {
             Model = "claude-sonnet-4.5",