fix(harness): reject empty old string in file edits

[wangxuefen88]

Summary

• reject null or empty old_string before counting occurrences
• prevent edit_file from hanging indefinitely
• add regression tests for empty and null values

Testing

• mvn -q -pl agentscope-harness -am test

Fixes #2446

[gemini-code-assist]

Caution

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[CLAassistant]

All committers have signed the CLA.

[oss-maintainer]

Code changes look good. The guard clause correctly prevents the infinite loop caused by countOccurrences on an empty string, and the regression tests cover both null and empty cases.

Note: CLA is not yet signed — the contributor needs to sign it before merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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