fix: populate mstraj velocity and acceleration

[Doribelove]

Summary

mstraj() returned a Trajectory with positions only, leaving its public qd and qdd properties as None. This change carries the analytic velocity and acceleration from each jtraj polynomial blend into the output, and assigns constant velocity and zero sampled acceleration to each linear segment. It also marks the time-based trajectory as istime=True and documents the returned fields and the instantaneous velocity changes when tacc=0.

The original position, time, arrival and segment metadata remain unchanged. In 50 generated cases, these fields matched upstream main exactly.

Related issue

Fixes #497.

Validation

• New tests reproduced the None result on upstream main; with the fix, tests/test_trajectory.py and tests/test_tools.py pass: 23 passed, 18 subtests passed on Python 3.12.
• The six-axis example in None value with variable "qd" and "qdd" in function "mstraj" return #497 now returns finite qd and qdd arrays with the same shape as q.
• ruff format --check and git diff --check pass. ruff check reports the same 15 pre-existing findings on both upstream main and this branch; this PR adds none.
• The full repository test command was attempted, but test_BaseRobot.py::test_ets_gripper2 timed out while robot_descriptions fetched the external YuMi model assets. No failure was observed in the trajectory tests.

Checklist

• PR title follows Conventional Commits.
• Added tests for blended and unblended multi-axis trajectories.
• New and changed code uses modern type hints.
• Updated the function docstring.
• Limited the change to mstraj and its trajectory tests.

The patch was developed with assistance from OpenAI Codex. The commit is signed off by 李永祺 doribelove@gmail.com.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (848114b) to head (8c47528).

Files with missing lines	Patch %	Lines
src/roboticstoolbox/tools/trajectory.py	0.00%	18 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #706   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        143     143           
  Lines      14269   14280   +11     
=====================================
- Misses     14269   14280   +11     

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