Refactor duplicated code in solver modules

[Copilot]

Description

Consolidates duplicated code patterns across kinematics solver modules into shared utility functions.

Changes:

• TCP handling: Added _get_tcp_as_numpy() helper to SolverCfg base class, used by all 6 solver configs
• Robot model reduction: Extracted build_reduced_pinocchio_robot() to solver_utils.py, replacing duplicate _get_reduce_robot() in PinocchioSolver and PinkSolver
• Iteration param validation: Added validate_iteration_params() utility, shared by PinocchioSolver and PytorchSolver
• Pinocchio FK: Added compute_pinocchio_fk() utility, replacing duplicate _get_fk() implementations

# Before: duplicated in each solver config
if isinstance(self.tcp, torch.Tensor):
    tcp = self.tcp.cpu().numpy()
else:
    tcp = self.tcp
solver.set_tcp(tcp)

# After: single helper in base class
solver.set_tcp(self._get_tcp_as_numpy())

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

N/A - refactoring with no functional changes

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

Original prompt

Find and refactor duplicated code

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