Update logfilter.py

[jangjangji]

• Add buffer.seek(0) after truncate to reset file pointer position
• Add isinstance and len check before accessing item[16] to prevent IndexError
• Improve robustness when processing malformed CSV log entries

Fixes #ISSUE_Number

What does this PR do?

This PR fixes two issues in the CsvFlatten class in gplogfilter:

1. Buffer pointer reset: Added buffer.seek(0) after truncate(0) to properly reset the StringIO file pointer position.
2. Index bounds checking: Added validation before accessing item[16] to prevent IndexError on malformed CSV rows.

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Type of Change

• [x] Bug fix (non-breaking change)
• [ ] New feature (non-breaking change)
• [ ] Breaking change (fix or feature with breaking changes)
• [ ] Documentation update

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Test Plan

• [x] Manual testing with production CSV log files
• [x] Verified gplogfilter correctly filters CSV logs with time range and pattern matching
• [ ] Unit tests added/updated
• [ ] Integration tests added/updated
• [ ] Passed make installcheck
• [ ] Passed make -C src/test installcheck-cbdb-parallel

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Impact

Performance:
No measurable impact expected.

User-facing changes:
None (internal logic improvement only).

Dependencies:
No changes.

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Checklist

• [x] Followed contribution guide
• [ ] Added/updated documentation
• [x] Reviewed code for security implications
• [ ] Requested review from cloudberry committers

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Additional Context

N/A

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CI Skip Instructions

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[jangjangji]

Looks good, ship it.

Thanks for reviewing!