Add tracing regression smoke tests

[geoffreyclaude]

Is your feature request related to a problem or challenge?

#14547 introduced the JoinSetTracer trait for tracing context propagation in spawned tasks, but was lacking integration tests. This means that any change to the core task spawning mechanism inside DataFusion, susceptible to break the tracing functionality, would go unnoticed.

Describe the solution you'd like

Unit tests should fail if the JoinSetTracer integration does not work as expected anymore. As a comprehensive complete test suite would pretty much require a full code coverage, a simple "smoke test" that fails if the JoinSetTracer cannot be properly registered should be a good first step.

Describe alternatives you've considered

Additional context

@ashdnazg in #15653 modified task spawning, and it was only by luck that @gabotechs caught the possible regression in #15653 (comment) .

[geoffreyclaude]

take