ref(core): Avoid boxing in DateUtils.nanosToDate (#5523)

* ref(core): Avoid boxing in DateUtils.nanosToDate

nanosToMillis already returns a primitive double, but the result was
stored in a boxed Double and then unboxed again via longValue(). Keep
the value primitive to drop the redundant allocation and unboxing on
this conversion, which runs whenever a SentryDate is turned into a
java.util.Date.

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>