Use Path.Join instead of Path.Combine to avoid silent-drop warnings

CodeQL flags Path.Combine because if a later argument is rooted, it
silently discards the earlier ones. Path.Join concatenates segments
literally and is the safer choice for these cases.

- E2ETestContext.ResolveSymlinks: use Path.Join when walking path
  components (the loop appends user-controlled directory names that
  realistically never start with a path separator on these temp dirs,
  but the safer API removes the risk regardless).
- HooksE2ETests: use Path.Join for the protected-file lookup.

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

Author: stephentoub

dotnet/test/E2E/HooksE2ETests.cs

@@ -165,7 +165,7 @@ await session.SendAsync(new MessageOptions
         // Strengthen: verify the actual deny behavior — the protected file was NOT
         // modified by the runtime even though the LLM tried to edit it. The pre-tool-use
         // hook denial blocks tool execution before it can mutate state.
-        var actualContent = await File.ReadAllTextAsync(Path.Combine(Ctx.WorkDir, "protected.txt"));
+        var actualContent = await File.ReadAllTextAsync(Path.Join(Ctx.WorkDir, "protected.txt"));
         Assert.Equal(originalContent, actualContent);
     }
 }

dotnet/test/Harness/E2ETestContext.cs

@@ -81,7 +81,7 @@ private static string ResolveSymlinks(string path)
             var resolved = root;
             foreach (var component in components)
             {
-                resolved = Path.Combine(resolved, component);
+                resolved = Path.Join(resolved, component);
                 try
                 {
                     var info = new DirectoryInfo(resolved);