Add regression test for unread subscription readiness across waits (backport #282)

[mergify]

Description

Add a cross-RMW regression test verifying that a subscription with an unread sample remains ready across repeated rmw_wait() calls.

The test does the following:

• Uses the public test_msgs/UnboundedSequences message, whose unbounded uint8 field exercises the buffer-aware Fast DDS path.
• Publishes a single sample and waits for the subscription to become ready.
• Calls rmw_wait() again without taking the sample and verifies that the subscription remains ready.
• Takes the sample and verifies its contents.
• Requires no external buffer backend or internal test package.

This reproduces the case where an executor observes a ready subscription but does not immediately take its sample, such as when a mutually exclusive callback group is temporarily unavailable.

Is this user-facing behavior change?

No. This PR adds a new test case.

Did you use Generative AI?

Yes. OpenAI GPT-5.6 was used to draft the changes in this PR.

Additional Information

This is to test the case fixed in ros2/rmw_fastrtps#900

---

This is an automatic backport of pull request #282 done by [Mergify](https://mergify.com).

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[ahcorde]

Pulls: ros2/rmw_fastrtps#902, #284
Gist: https://gist.githubusercontent.com/ahcorde/289a9448d87bc4c769ce36ccf9f3832b/raw/3fabceb64949ed68ec56ed74b0029f03fdad4708/ros2.repos
BUILD args: --packages-above-and-dependencies rmw_fastrtps_cpp test_rmw_implementation
TEST args: --packages-above rmw_fastrtps_cpp test_rmw_implementation
ROS Distro: lyrical
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/19906

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