Fix: Include slave_id when moving tasks

[wiegandm]

What

Default move_task() to the master scanner by sending slave_id="" in GMPv224-derived clients (including GMPv227) and GMPNext. Reject an explicit None rather than generating an invalid request. Update docstrings and tests for default, empty, nonempty, and None values.

Why

GMP requires a slave_id attribute for move_task; omitting it caused gvmd to return 400 instead of moving the task to the master scanner.

References

• https://docs.greenbone.net/API/GMP/gmp-22.8.html#command_move_task
• https://github.com/greenbone/gvmd/blob/4caf93f5a9f8f76c8b509f7401b51d1085e9b51b/src/gmp.c#L29451-L29470

Checklist

• I have added tests for the changes
• I have used the following LLMs/AI tools in this pull request: pi coding agent (openai-codex/gpt-6-sol) to implement and review the fix and prepare the local commit and PR description

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

⚠️: No snapshots were found for the head SHA be71c3c.

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Scanned Files

None

[github-actions]

Conventional Commits Report

Type	Number
Bug Fixes	2

🚀 Conventional commits found.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (c5e320f) to head (be71c3c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1390   +/-   ##
=======================================
  Coverage   98.40%   98.40%           
=======================================
  Files         106      106           
  Lines        6886     6888    +2     
  Branches     1244     1244           
=======================================
+ Hits         6776     6778    +2     
  Misses         77       77           
  Partials       33       33           

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