Stderr with Merged Streams Test

# Stderr with merge_streams Test

This page tests how the stderr warning feature handles the `merge_streams` option in myst-nb.

## Test Case 1: merge_streams=False (default) - Separate stderr class

When merge_streams is False, stderr gets its own `.output.stderr` element. Our feature SHOULD collapse this.

    print("Normal output")
    warnings.warn("This is a warning")

    Normal output

    UserWarning: This is a warning
      warnings.warn("This is a warning")

## Test Case 2: merge_streams=True - Combined in .output.stream

When merge_streams is True, myst-nb combines stdout and stderr into a single `.output.stream` element. Our feature should NOT collapse this since there's no `.output.stderr` class.

    print("Normal output")
    warnings.warn("This is a warning")

    Normal output
    UserWarning: This is a warning
      warnings.warn("This is a warning")

## Test Case 3: Only stderr with merge_streams=True

Even when merge_streams is True, if a cell ONLY produces stderr (no stdout), it should still use `.output.stderr` class and our feature SHOULD collapse it.

    warnings.warn("Only a warning, no stdout")

    UserWarning: Only a warning, no stdout
      warnings.warn("Only a warning, no stdout")

## Expected Behavior Summary

- Test Case 1: Should show "⚠ Warnings" button (stderr is separate)
- Test Case 2: Should display normally without button (merged streams)
- Test Case 3: Should show "⚠ Warnings" button (only stderr)

Key insight: When `merge_streams=True`, myst-nb combines stdout and stderr into a single `.output.stream` element. Our feature only targets `.output.stderr` elements, so merged streams will display normally. This is the correct behavior because:

- Merged streams indicate the user WANTS stdout and stderr together
- We shouldn't collapse output that contains both normal output and warnings
- Only pure stderr warnings should be collapsible