Honor Universal Robots model-specific update rates

[Daniiiil1]

Summary

• let UR model-specific controller parameters override Clearpath generic defaults
• preserve generic controller keys that are not present in the UR driver file
• add a regression test for the UR5 CB3 125 Hz update rate

Validation

• Python bytecode compilation passed for the modified module and test
• focused unit harness passed with mocked ROS/package dependencies
• git diff --check passed

A full ROS/ament test environment was not available locally.

Closes #346.

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