perf: Avoid boxing in doubleToBigDecimal timestamp serialization (#5551)

* perf: Avoid boxing in doubleToBigDecimal timestamp serialization

Change DateUtils.doubleToBigDecimal to take a primitive double instead of a
boxed Double, and route the four duplicated private copies (ProfileChunk,
ProfileMeasurementValue, SentrySample, SentrySpan) through it. Callers that
hold a primitive double timestamp no longer autobox on every serialization,
and the duplicated helpers are consolidated into one. Behavior is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* changelog

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>