Fix race condition when creating isolated component container (backport #3223)

[mergify]

Description

Fix race condition when creating isolated component container

What

The isolated component_container created a throwaway ComponentManager solely to read the thread_num parameter, then destroyed it and re-created the real ComponentManagerIsolated. This briefly advertised the container's ~/_container/load_node service and tore it down, racing LoadComposableNodes clients and silently dropping load requests.

Changes

• component_container.cpp: construct the manager once, directly as its final type. Isolated managers are now built with num_threads == 0 ("auto").
• component_manager_isolated.hpp: in add_node_to_executor, num_threads_ == 0 now resolves the thread_num parameter itself (defaults to hardware concurrency), so the container no longer needs to read it up front.
• Added a regression test (isolated_multi_threaded_auto_thread_num) covering the isolated multi-threaded "auto" path.

Notes

• ABI-safe: only an executable and an inline template method body change; ComponentManager's layout is untouched.
• Behavior preserved, including -p thread_num:=N for isolated multi-threaded/events executors.
• Cleanly backportable to lyrical (identical baseline).

Fixes #3222

Is this user-facing behavior change?

Did you use Generative AI?

Additional Information

---

This is an automatic backport of pull request #3223 done by [Mergify](https://mergify.com).

[skyegalaxy]

Pulls: #3228
Gist: https://gist.githubusercontent.com/skyegalaxy/e19815dc0cd2c09ede3fe2deaf7d541e/raw/f2f7f1d4800ec4ef046a027d85ea6f288095128f/ros2.repos
BUILD args: --packages-above-and-dependencies rclcpp
TEST args: --packages-above rclcpp
ROS Distro: lyrical
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20076

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

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