Write tests for streaming_merge

### Is your feature request related to a problem or challenge?

With #6874, `physical_plan::merge::streaming_merge` is now public.

Without some care, the change may be lost in some future refactoring (e.g. if the code is moved to another module, for example, that is not pub).

### Describe the solution you'd like

Write a test, perhaps as an example of how to use this API, in: https://github.com/apache/arrow-datafusion/tree/main/datafusion-examples/examples

Might be able to adapt some of the existing code in https://github.com/apache/arrow-datafusion/blob/main/datafusion/core/tests/fuzz_cases/merge_fuzz.rs or https://github.com/apache/arrow-datafusion/blob/main/datafusion/core/benches/sort.rs

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start in datafusion-examples/examples and review the existing merge_fuzz.rs and sort.rs files for patterns that can exercise the public physical_plan::merge::streaming_merge API. Add an example or test demonstrating its use, and confirm the test preserves coverage of the public API.

Written by the indexing model from the issue text.