Write ExpectedExitCode marker in GenerateMarkerFiles (#124582)

`GenerateMarkerFiles` did not emit the expected-exit-code marker, so
merged/standalone test payloads could miss `ExpectedExitCode.txt`. This
change adds marker generation directly in the existing marker pipeline.

- **Scope**
- Updates `src/tests/Directory.Build.targets` in `GenerateMarkerFiles`.

- **Behavior change**
- Adds a marker entry for
`$(IntermediateOutputPath)\ExpectedExitCode.txt`.
  - Writes `$(ExpectedExitCode)` as file content.
  - Applies only when:
    - `$(ExpectedExitCode)` is set
    - test is merged runner or standalone runner
    - `$(CLRTestKind)` is not `SharedLibrary`

- **Implementation (minimal diff)**
  ```xml
  <MarkerFile Include="$(IntermediateOutputPath)\ExpectedExitCode.txt"
Condition="'$(ExpectedExitCode)' != '' and
('$(IsMergedTestRunnerAssembly)' == 'true' or
'$(BuildAllTestsAsStandalone)' == 'true') and '$(CLRTestKind)' !=
'SharedLibrary'">
    <Type>$(ExpectedExitCode)</Type>
  </MarkerFile>
  ```

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Author: Copilot

src/tests/Directory.Build.targets

@@ -434,6 +434,9 @@
       <MarkerFile Include="$(IntermediateOutputPath)\$(AssemblyName).NoMonoAot" Condition="'$(MonoAotIncompatible)' == 'true'">
         <Type>NoMonoAot</Type>
       </MarkerFile>
+      <MarkerFile Include="$(IntermediateOutputPath)\ExpectedExitCode.txt" Condition="'$(ExpectedExitCode)' != '' and ('$(IsMergedTestRunnerAssembly)' == 'true' or '$(BuildAllTestsAsStandalone)' == 'true') and '$(CLRTestKind)' != 'SharedLibrary'">
+        <Type>$(ExpectedExitCode)</Type>
+      </MarkerFile>
       <Content Include="@(MarkerFile)" CopyToOutputDirectory="PreserveNewest" />
     </ItemGroup>