CNDB-11460 Fix cqlsh unicode tests in TestCqlshUnicode by handling UTF-8 characters that are split across reads.

[djatnieks]

What is the issue

CNDB-11460 - several cqlsh (jvm dtest) tests in TestCqlshUnicode consistently fail, with an error like:

E           UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 4-5: unexpected end of data

What does this PR fix and why was it fixed

I took a look at the test code and asked Augment for suggestions - it recommended adding some code to handle UTF-8 characters that may be split across reads. I tried the code and it looks like it does the job.

[github-actions]

Checklist before you submit for review

• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-1561 rejected by Butler

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1 new test failure(s) in 2 builds
See build details here

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Found 1 new test failures

Test	Explanation	Branch history	Upstream history
o.a.c.u.b.BinLogTest.testTruncationReleasesLogS...	regression	🔴🔴	🔵🔵🔵🔵🔵🔵🔵

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Found 4 known test failures