Fix Servo Jacobian fallback without IK solver

[dlanov]

Description

Fixes #3789.

MoveIt Servo already has a direct-Jacobian fallback in jointDeltaFromIK() when no IK solver is configured. However, TWIST and POSE commands could not reach that path because getIKSolverBaseFrame() and getIKSolverTipFrame() returned std::nullopt whenever the planning group had no IK solver.

This change allows solver-less kinematic chain groups to use the same base and tip frames that RobotState::getJacobian() uses internally:

• base frame: parent link of the group's first joint
• tip frame: last link in the group

Groups with an IK solver retain the existing behavior and continue using the solver-provided frames.

For non-chain groups without an IK solver, the helpers continue to fail safely with std::nullopt rather than choosing an ambiguous frame.

Regression coverage verifies:

• solver-less chain groups resolve the expected base and tip frames;
• non-chain groups without an IK solver still fail safely;
• TWIST commands reach the existing Jacobian fallback;
• POSE commands reach the existing Jacobian fallback.

Testing

Validated with an incremental moveit_servo build:

• colcon build --packages-select moveit_servo — passed
• targeted no-IK-solver regression tests — 4/4 passed
• complete moveit_servo_utils_test — 18/18 passed
• pre-commit on all changed files — passed
• git diff --check — passed

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation
• Document API changes relevant to the user in MIGRATION.md
• Create tests, which fail without this PR
• Include a screenshot if changing a GUI

Summary by CodeRabbit

• New Features

  • Twist and pose commands can now operate for unambiguous kinematic-chain groups even when no IK solver is configured.
  • Servo automatically determines suitable base and tip frames from the robot’s chain structure.

• Bug Fixes

  • Improved handling and diagnostics when frame information cannot be determined.
  • Branched or empty groups now fail safely instead of producing invalid command results.

• Tests

  • Added coverage for frame fallback behavior and Jacobian-based command handling without an IK solver.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

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Run ID: 9334a21c-e22d-4eea-857a-5f003898f5e4

📥 Commits

Reviewing files that changed from the base of the PR and between 8fcb5d2 and 21b5047.

📒 Files selected for processing (4)

• moveit_ros/moveit_servo/include/moveit_servo/utils/common.hpp
• moveit_ros/moveit_servo/src/servo.cpp
• moveit_ros/moveit_servo/src/utils/common.cpp
• moveit_ros/moveit_servo/tests/test_utils.cpp

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📝 Walkthrough

Walkthrough

This change enables solver-less chain groups to resolve Jacobian-compatible base and tip frames. Twist and Pose command handling now accepts these frames, reports ambiguous frame failures, and tests finite nonzero joint deltas.

Changes

Solver-less Jacobian command handling

Layer / File(s)	Summary
Chain-based frame resolution moveit_ros/moveit_servo/include/moveit_servo/utils/common.hpp, moveit_ros/moveit_servo/src/utils/common.cpp, moveit_ros/moveit_servo/tests/test_utils.cpp	Frame helpers fall back to the first joint’s parent link and the group’s last link for chain groups without IK solvers. Empty and non-chain groups return std::nullopt.
Twist and Pose Jacobian command flow moveit_ros/moveit_servo/src/servo.cpp, moveit_ros/moveit_servo/tests/test_utils.cpp	Servo diagnostics accept solver-derived or chain-derived frames. Twist and Pose tests verify finite, nonzero joint deltas for solver-less chain groups.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 21b50

The change enables TWIST and POSE commands to use the existing Jacobian fallback for solver-less chain groups while preserving safe behavior for non-chain groups; no actionable merge-blocking risk remains beyond normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant Servo
  participant FrameHelpers
  participant RobotState
  participant JacobianFallback
  Servo->>FrameHelpers: resolve planning base and tip frames
  FrameHelpers-->>Servo: return chain frames
  Servo->>RobotState: process Twist or Pose command
  RobotState->>JacobianFallback: compute joint deltas from Jacobian
  JacobianFallback-->>Servo: return finite nonzero joint deltas

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the Servo Jacobian fallback change for configurations without an IK solver.
Linked Issues check	✅ Passed	The changes satisfy issue #3789 by enabling solver-less chain groups to use the existing direct Jacobian path while preserving safe failure for non-chain groups.
Out of Scope Changes check	✅ Passed	The changes are limited to frame fallback logic, related diagnostics, documentation, and regression tests required by issue #3789.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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