ref(core): Reduce unnecessary boxing and redundant null checks (JAVA-554) (#5520)

* ref(core): Use static compare and drop redundant null checks

Replace boxed Long.valueOf(...).compareTo(...) with Long.compare(...),
which avoids the unnecessary boxing. Also remove the redundant != null
checks that precede an instanceof, since instanceof already returns
false for null.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ref(core): Remove unnecessary boxing (JAVA-554)

Replace Integer.valueOf/Double.valueOf boxing with primitives or the
appropriate parse method. String.format takes the primitives directly,
the double conversions only need a cast, and the version check parses
straight to a primitive double via Double.parseDouble.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* changelog

* ref(core): Drop redundant StringBuilder in hashing helper (JAVA-554)

Return the hex string directly instead of wrapping it in a StringBuilder
only to immediately call toString().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ref(core): Use StandardCharsets.UTF_8 and tidy comments (JAVA-554)

Replace Charset.forName("UTF-8") with the StandardCharsets constant,
which avoids the lookup and cannot throw a checked exception. Also
collapse the leftover hashing comments into one.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>