Fix set_simulation_state STOPPED timeout

[nekomario28]

Fork-local evidence carrier for upstream gazebosim#950 / issue gazebosim#919. Do not merge this fork PR.

Current base: upstream-aligned ros2 at 2ee8a5c716d1838de0b7ac817638ad5e9304955e.
Current head: 2e830dd25172f1a066c04ff85e79cf8389cf91b7.

Scope

• arm reset detection before issuing reset requests, avoiding a lost completion notification between action and wait;
• make ResetSimulation use the same arm -> request -> wait contract;
• preserve pause during canonical-link state synchronization for released gz-sim versions;
• implement STOPPED as pause-and-confirm, reset-with-pause, then require reset detection and paused && iterations == 0;
• add public-interface PLAYING -> STOPPED regression coverage and restore PLAYING afterward.

The attempted PAUSED -> STOPPED / repeated STOPPED -> STOPPED expansion was reverted because the fixture incorrectly expected PAUSED immediately after a STOPPED reset, while the public classifier still reports STOPPED for paused && iterations == 0.

Validation

Current-tree fork ROS2 CI run 32841870243:

• Pre-commit: PASS (job 97782959967)
• ros_gz CI (ubuntu:resolute, rolling) Build and Test: PASS (job 97782959680)

The run event is pull_request. Its synthetic merge tree is identical to the current head tree, so this is current-tree validation; it should not be described as exact-head-commit execution.

Earlier Jazzy / Gazebo Harmonic (gz-sim8) minimized candidate, run 32483225071 / job 96773957402:

• build: PASS
• ros_gz_sim tests: PASS
• real gzserver PLAYING -> STOPPED: PASS

Earlier subtractive evidence:

• removing pause preservation: build/tests PASS, real gzserver FAIL with Timed out while trying to stop simulation (32470884388 / 96737207002);
• collapsing STOPPED to one pause+reset request while retaining pause preservation and reset arming: build/tests PASS, real gzserver FAIL at the final STOPPED condition (32479212955 / 96761787242).

Backport

The bug was reproduced and fixed on ROS 2 Jazzy + Gazebo Harmonic / gz-sim8, so a Jazzy backport remains justified after the mainline change is accepted.

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