fix(robot): the first line of print(robot) names the class and the model

[petercorke]

print(robot) began with a hard-coded ERobot: for every robot that is not a DHRobot, which is the name of a class that no longer exists, and so said the wrong thing for a Robot, a Robot2 and a URDF model.

Behaviour change

The first line of the display of a robot is now X: name or X.Y: name, where X is the kind of robot (Robot, Robot2, DHRobot or URDFRobot) and Y is the class of the model when it is a subclass, for example:

Robot.Planar_Y: Planar-Y, 6 joints (RRRRRR), 2 branches
URDFRobot.Panda: panda (by Franka Emika), 7 joints (RRRRRRR), 1 gripper, geometry, collision
DHRobot.Puma560: Puma 560 (by Unimation), 6 joints (RRRRRR), dynamics, geometry
Robot: arm, 1 joints (R)

A subclass written by a user shows as Robot.Mine: .... Code that parses the first line of the display would be affected; the rest of the display is unchanged.

Changes

• BaseRobot._class_label() gives the label, and is used by BaseRobot.__str__ and DHRobot.__str__, which each had a copy of the header. The README example is updated.
• tests/test_robot_label.py (5 tests): an ETS model, a URDF model, a DH model, the classes themselves (Robot, DHRobot, Robot2), and a user's subclass.

Full suite: only the 20 test_servo_half_turn failures from the stale compiled extension in my worktree; mypy clean. Written with AI assistance (Claude Code).

🤖 Generated with Claude Code

[codacy-production]

Not up to standards ⛔

🔴 Issues 7 high

Alerts:
⚠ 7 issues (≤ 0 issues of at least minor severity)

Results:
7 new issues

Category	Results
Security	7 high

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🟢 Metrics 11 complexity · 0 duplication

Metric	Results
Complexity	11
Duplication	0

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (eee9488) to head (99f99b6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/robot/BaseRobot.py	0.00%	7 Missing ⚠️
src/roboticstoolbox/robot/DHRobot.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #862   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        146     146           
  Lines      16846   16851    +5     
=====================================
- Misses     16846   16851    +5     

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