[reginaldalfret]

Summary

This PR addresses two edge-case issues in JsonGraphEngine:

1. Sub-second step division by zero: When serializing the explain metadata block, graphDatapoints was computed as graphLines * ((end - start) / (config.step / 1000) + 1). When queries use sub-second step sizes (such as 500ms, 100ms, etc., which are valid in Step.allowedStepSizes), integer division (config.step / 1000) truncates to 0, resulting in ArithmeticException: / by zero. Dividing directly by config.step in milliseconds avoids division by zero while preserving the exact datapoint count.
2. Large step integer overflow: config.step was cast to Int via .asInstanceOf[Int]. For large step sizes (> 24.8 days in milliseconds), this overflows to negative values, causing the values loop (timestamp += step) to decrease and loop infinitely. Keeping step as Long matches CsvGraphEngine and prevents overflow.
3. Test additions: Added regression test coverage for explain stats, sub-second step handling, large step rollover, and stats.json in JsonGraphEngineSuite. Also normalized newline comparison in CsvGraphEngineSuite for cross-platform consistency.

Verification

All 335 tests in atlas-chart pass:

• atlas-chart/test (335 passed, 0 failed)
• atlas-chart/scalafmtCheck passed
• checkLicenseHeaders passed