fix(filegen): select random line on each run and add file caching

[Dylan-M]

Proposed Change

Fixed the filegen generator to select a single random line from each file on each work cycle, and added file caching to improve performance.

Changes:

• Fixed readAndWriteFile() to read the entire file into memory, collect non-empty lines, and select one at random (instead of writing all lines sequentially)
• Added math/rand import for random line selection
• Implemented file caching with thread-safe RWMutex to avoid repeated disk reads
• Files are cached with automatic 1-minute TTL expiration
• Extracted file reading logic into readFileLines() helper function
• Updated documentation to clarify the random line selection behavior
• Added Caching section to documentation explaining cache behavior

Checklist

• Changes are tested
• CI has passed