overflow in toFileSize causing incorrect log rotation limits

[jmestwa-coder]

Summary

Fixes an integer overflow in OptionConverter::toFileSize that can break log rotation.

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Problem

Extremely large values (e.g. "9999999999999999999999GB") can overflow a signed long, become negative, and then convert to a very large size_t.
This results in log rotation never triggering.

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Fix

• Use strtoll for parsing
• Reject overflow, negative values, and out-of-range results
• Safely fall back to defaultValue

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Test

Adds a deterministic test to verify large inputs no longer produce unrealistic file size limits.