Add test for --unload-on-kill spawner lock behavior

[naitikpahwa18]

This PR adds a regression test to verify that a spawner started with
--unload-on-kill does not block other spawners while waiting for an
interrupt.

The test launches one spawner in the background and ensures a second
spawner can start without being delayed, covering the locking fix
introduced in #2559

Closes #2566

[Copilot]

Pull request overview

Adds a C++ regression test in controller_manager to ensure a spawner started with --unload-on-kill does not hold the global spawner lock while waiting for interrupt, allowing other spawners to start promptly (covers the fix from #2559; closes #2566).

Changes:

• Add unload_on_kill_does_not_block_other_spawners test that runs one spawner in the background with --unload-on-kill.
• Measure that a second spawner invocation completes within a bounded time, indicating it wasn’t blocked on the lock.

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[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.43%. Comparing base (c569292) to head (d955886).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
controller_manager/test/test_spawner_unspawner.cpp	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3035      +/-   ##
==========================================
+ Coverage   89.39%   89.43%   +0.03%     
==========================================
  Files         157      157              
  Lines       18783    18794      +11     
  Branches     1510     1511       +1     
==========================================
+ Hits        16792    16808      +16     
+ Misses       1369     1366       -3     
+ Partials      622      620       -2     

Flag	Coverage Δ
unittests	89.43% <91.66%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	96.00% <91.66%> (-0.13%)	⬇️

... and 3 files with indirect coverage changes

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