feat(hardware_interface): implement dynamic command toggling for MockHardware

[shlok-mehndiratta]

Closes #3214

While working on some controller tests, I realized that the GenericSystem mock always mirrors commands to states perfectly. This makes it hard to test how high-level controllers or state machines react when the hardware actually stops responding or gets stuck (e.g., communication loss or a physical state-lock).

This PR adds a ~/set_command_propagation service (using std_srvs/SetBool) to the GenericSystem mock. It lets you manually "freeze" the command propagation at runtime:

• data: false: The mock stops updating its state and stays at its last value, ignoring new commands.
• data: true: Resumes normal mirroring.

Implementation Notes

• Thread Safety: I used a std::atomic<bool> with memory_order_acquire/release for the toggle. This keeps the real-time read() loop lock-free and safe from the asynchronous service thread without adding mutex overhead.
• Scope: This is strictly for the mock_components/GenericSystem plugin. I haven't tested this on physical hardware since it's a simulation-only feature, and it shouldn't have any side effects on real drivers.
• Testing: I've updated the TestableResourceManager helper to support injecting an executor, which allowed me to verify the service logic in a standard unit test.

I've verified this both with a new test case (toggle_command_propagation_service) and manually with ros2 service call. All 29 existing tests pass.

Hope this helps anyone else trying to simulate hardware gremlins in their tests!

Verification Screenshot

(I'll attach a screenshot here showing the join states freezing after the service call)

[codecov]

Codecov Report

❌ Patch coverage is 75.86207% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.06%. Comparing base (c178e16) to head (d0d5b7d).

Files with missing lines	Patch %	Lines
...rface/test/mock_components/test_generic_system.cpp	71.73%	10 Missing and 3 partials ⚠️
...e_interface/src/mock_components/generic_system.cpp	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3241      +/-   ##
==========================================
- Coverage   89.10%   89.06%   -0.04%     
==========================================
  Files         160      160              
  Lines       19753    19821      +68     
  Branches     1597     1601       +4     
==========================================
+ Hits        17600    17653      +53     
- Misses       1486     1498      +12     
- Partials      667      670       +3     

Flag	Coverage Δ
unittests	89.06% <75.86%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...terface/include/mock_components/generic_system.hpp	100.00% <ø> (ø)
...e_interface/src/mock_components/generic_system.cpp	81.18% <91.66%> (+0.26%)	⬆️
...rface/test/mock_components/test_generic_system.cpp	97.00% <71.73%> (-2.76%)	⬇️

... and 12 files with indirect coverage changes

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