test(ros): fail the interop job when it runs no tests

[YuanYuYuan]

Summary

The ROS interop step captured nextest's output with complete and never printed it, so a passing job logged the command and then ✅ All ROS 2 <distro> tests passed! with nothing in between.

That banner could not be falsified. nextest exits 0 when it runs zero tests, and every interop test additionally returns early — still passing — when check_ros2_available says no. Nothing in the log distinguished "137 interop tests passed against rmw_zenoh_cpp" from "the binary matched no tests".

Key Changes

Print the captured output, and require a nextest summary reporting a non-zero count. A run that produces no summary line, or reports 0 tests run, is now a hard error rather than a pass.

What fails without this

Before, the entire step output between the echoed command and the success banner was empty. After, the same job prints every test and its own assertion:

Starting 137 tests across 25 binaries (3 tests skipped)
 Summary [  50.585s] 137 tests run: 137 passed, 3 skipped
137 ROS interop tests ran against rmw_zenoh_cpp.

The gate was checked in all three directions before being relied on: a 57-test summary passes, a 0 tests run summary errors, and absent output errors.

Notes

Split out of #250, where it rode along because the swallowed output was found while gathering evidence for that fix. It is CI hygiene and has nothing to do with subscriber re-entrancy.

Breaking Changes

None.