Fix 6DOF arm example (backport #804)

[mergify]

Thanks to debugging by @catarina-p and myself, we found issues with the 6DOF arm example.

Turns out the controller kept extrapolating forever past the final point, meaning that instead of slowing down from positive to zero velocity, the velocities kept going negative further and further.

Before this fix:
https://github.com/user-attachments/assets/af815279-818a-4922-837d-94b0208035cc

After this fix:
https://github.com/user-attachments/assets/70f4796e-38e1-4ba5-b2f2-5e402c324f2c

I also did some cleanup of the trajectory time calculations since the final point's time was not exactly 3.0 as expected.

Edit: As a side note, I am somewhat surprised that this example doesn't just use the regular joint trajectory controller that is already available in ros2_control. Although it's a nice example for people writing their own controllers.

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This is an automatic backport of pull request #804 done by Mergify.

[mergify]

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	both modified:   example_7/controller/r6bot_controller.cpp
	both modified:   example_7/reference_generator/send_trajectory.cpp

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