Unify transmission tests using shared minimal robot URDF in test assets

[naitikpahwa18]

This PR extends the shared minimal_robot_urdf in ros2_control_test_assets with transmission definitions (Simple, Differential, and FourBar) and updates the transmission loader tests to use this canonical URDF instead of locally embedded XML strings. It also introduces reusable transmission snippets (including an invalid variant) for parser-related tests. The goal is to consolidate test URDF definitions into a single fully-featured robot description, reduce duplication, and align with the architectural intent discussed in issue #648.

[Copilot]

Pull request overview

This PR consolidates transmission-related test URDF definitions by extending the shared ros2_control_test_assets::minimal_robot_urdf with multiple transmission definitions (Simple, Differential, FourBar) and updating transmission loader tests to use this canonical URDF instead of embedding XML strings.

Changes:

• Extend minimal_robot_urdf test asset with ros2_control transmission tags for Simple, Differential, and FourBar transmissions.
• Add reusable “classic URDF transmission” snippets (minimal_robot_transmissions and an invalid variant) for parser-focused tests.
• Update transmission loader tests to consume minimal_robot_urdf and (where needed) locate the correct transmission by type.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
transmission_interface/test/simple_transmission_loader_test.cpp	Switch FullSpec test URDF source to shared minimal_robot_urdf.
transmission_interface/test/differential_transmission_loader_test.cpp	Switch FullSpec test to shared minimal_robot_urdf and find the Differential transmission by type.
transmission_interface/test/four_bar_linkage_transmission_loader_test.cpp	Switch FullSpec test to shared minimal_robot_urdf and find the FourBar transmission by type.
ros2_control_test_assets/include/ros2_control_test_assets/descriptions.hpp	Add transmission definitions/snippets and include them in minimal_robot_urdf.

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[christophfroehlich]

Thanks for taking this old issue.

Please consider our contributing guidelines and have a look at the failing pre-commit job.

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.29%. Comparing base (cb9a983) to head (c28dd85).

Files with missing lines	Patch %	Lines
...ace/test/differential_transmission_loader_test.cpp	81.81%	0 Missing and 2 partials ⚠️
...test/four_bar_linkage_transmission_loader_test.cpp	81.81%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3031      +/-   ##
==========================================
+ Coverage   89.28%   89.29%   +0.01%     
==========================================
  Files         158      158              
  Lines       19296    19308      +12     
  Branches     1560     1568       +8     
==========================================
+ Hits        17228    17241      +13     
+ Misses       1424     1422       -2     
- Partials      644      645       +1     

Flag	Coverage Δ
unittests	89.29% <82.60%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...interface/test/simple_transmission_loader_test.cpp	84.21% <100.00%> (-0.28%)	⬇️
...ace/test/differential_transmission_loader_test.cpp	92.00% <81.81%> (-2.12%)	⬇️
...test/four_bar_linkage_transmission_loader_test.cpp	92.00% <81.81%> (-2.12%)	⬇️

... and 4 files with indirect coverage changes

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[christophfroehlich]

LGTM