Validate markovchainSequence length argument

[spedygiorgio]

Summary

• validate the requested length in markovchainSequence and guard against invalid values
• ensure the R implementation creates character sequences safely for zero-length requests
• add regression tests covering validation failures and zero-length output

Testing

• Rscript -e 'testthat::test_dir("tests/testthat")' (fails: Rscript is not available in the execution environment)

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https://chatgpt.com/codex/tasks/task_e_68f6aa0837f883209ee57f7167f7c76e

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.66%. Comparing base (b1562a7) to head (6b5e11d).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   45.56%   45.66%   +0.09%     
==========================================
  Files          24       24              
  Lines        3803     3810       +7     
==========================================
+ Hits         1733     1740       +7     
  Misses       2070     2070              

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